Hi,

I'm restoring a Crosley 60 which has 5 tubes to do the audio output circuit !!!!
A pair of 6J5-6AC5 and another pair of 6J5-6AC5 as push-pull amplifier, and a third 6J5 used as a phase inverter.

I'm not impress at all. The sound is not bad, but there is no bass, and not as much power as my other radios that has only a single 6F6 as audio output.

I have another Crosley that has a pair of pentodes 41M driven by a 6J5, and the audio is very powerfull.

So pentodes are better than triodes in audio amplifiers ?

Well is it normal that a push-pull of triodes like 6AC5 doens't drive power as much as a single pentode or push-pull pentodes ?

Pentodes typically have more gain. Triodes tend to sound more natural, though they can have a lot of bass. I temporarily switched my Crosley 1117 to 6N6 tubes as opposed to 6K6, which requires bypassing the cathode bias resistor. I'd say that gain is similar; perhaps a bit less. Bass is plenty.

I can't find your schematic here, so I can't recommend exactly what to do. Though Crosley typically designed very good radios, some radios can be designed poorly, though you'd think that with push-pull you'd have plenty of bass.

For starters, take a look at your speaker. Is it stiff? Is it REALLY stiff? This can be remedied. Old stiff paper speaker cones can be revitalized with neatsfoot oil to which a drop or two of dish detergent is added for surface tension breaking purposes. Fill a small bottle cap with the oil and add the detergent. Paint this onto the cone. The cone will soften right away. More permanent de-brittling effects take place in weeks or months.

Some cabinets aren't well designed acoustically. My Silvertone phonograph (I think it's model 5731) is this way. The speaker is moving all over the place inside, but the cabinet just doesn't have good acoustic characteristics. It's long and open-backed. Deep cabinets don't produce bass well. Also, the motorboard has large cut-outs beneath it to allow space for the motor. If you close the lid and place it at one end of a long room on the floor, bass will be quite nice about halfway across the room. If you place it up on a table or bureau, facing sideways in the room, bass will be almost nonexistent.

Within the amplifier grid and plate resistors will reduce bass if too low in value. With some tubes, such as the 6V6, you shouldn't increase the value of the grid bias resistor above its recommended value (470K), as grid current can run away, but with others (input audio) you can. With push-pull you must consider voltage dividing resistors and not upset the gain division for the phase inverter, etc. Increasing the value of a plate resistor from, say, 200K to 470K, or even 1Meg in some cases, can greatly increase bass. Increasing the resistance can reduce treble somewhat, as it does adjust the time constant of the circuit and somewhat broad-tune it.

The grid bias resistor of the input tube can, depending on the tube, sometimes be increased as high as 10Meg. Resistances around this value are used for triodes whose grids are self biased (grid leak method). Many amplifiers from the early to mid-30s use fixed bias where the bias voltage is obtained from a DC bias point in the power supply, and these amplifiers also often use lower value bias resistors (from 500K to 1Meg). Improvement can sometimes be obtained by the self-bias method with higher resistances. Care should be exercised, however, to be sure that the original set-up isn't better for hum purposes.

The volume control is also sometimes a source of bass supression. Controls around 500K work for radios with good driving power, but sometimes choke bass excessively, especially if fed with small value capacitors. Capacitors are another concern. Increasing the value of audio bypass capacitors will increase bass to a degree. Care should be exercised so as to not swamp the amplifier with low bass, rumble (from phonograph records), or AVC voltage changes, as this can wipe out the audio for brief periods.

It is best, first, to be sure that everything is in good working order. If everything seems to be fine, modifications can be made within reason that will improve bass response. All of the above component modifications will also increase gain.

Also, the 6N6 contains the two triodes that are separate in your radio, and the two of them are similar in gain to a pentode, though the 6AC5 itself has less gain that a pentode.

Some triode output amplifiers are pentode drive, too, to make up for gain reduction. The use of pentodes in the pre-amplifier section doesn't distort the frequency response as much as in the output, so you still get the natural triode sound.

T

Thanks Thomas ,

Here is my schematic below.

The speaker is really stiff indeed.
I will try to soften it with the neatfoot oil as you said.

Also i will try to install a lid behind the cabinet, that might help to improve the bass response.

BTW the 33K resistors on cathodes of V3 and V5 were not present on the original schematic, but my set had them installed, probably at the factory (old solder).

It's like i have a considerable drop of voltage after the field coil. 340v on the first cap, 240v on the second cap.

Hi Vianney

You won't get a lot out of that.. V4 is a phase inverter. No gain. V3 and V5 are cathode followers, no voltage gain there either. V2 through V6 could be replaced by a pair of 6F6's with rewiring. They could use Philco's method of phase inversion which doesn't require an extra tube or a 6AD7G in place of 1 - 6F6.

Norm

:Thanks Thomas ,
:
:Here is my schematic below.
:
:The speaker is really stiff indeed.
:I will try to soften it with the neatfoot oil as you said.
:
:Also i will try to install a lid behind the cabinet, that might help to improve the bass response.
:
:BTW the 33K resistors on cathodes of V3 and V5 were not present on the original schematic, but my set had them installed, probably at the factory (old solder).
:
:It's like i have a considerable drop of voltage after the field coil. 340v on the first cap, 240v on the second cap.
:
:
:

This looks similar to what I'd have if I plugged two 6N6 tubes into my Crosley 1117, except without the phase inverter. I get plenty of gain, power, and bass from my Crosley when I use the 6N6 tubes in place of the 6K6 tubes. Each 6N6 contains the equivalent of a 6AC5 and 6J5.

To increase bass and power, for starters, I'd increase the 6SQ7 plate resistor to between 500K and 1Meg. You might also increase the grid bias resistors of the 6J5 drivers to 500K (470K).

Most everything else seems conventional, though not quite leaning in the direction of a circuit that would have good booming bass. The 6SQ7 has a nice wide open grid circuit, though you could increase the 4.7Meg grid resistor all the way to 10Meg if you wanted. The volume control is a good value for full bass reproduction and gain.

I would be sure that the output transformer was the original one, and that someone didn't install an incorrect replacement. The DC resistance should be in the neighborhood of 500 ohms. Someone put an incorrect replacement output transformer in a Silvertone radio I own that uses a 6B5 (6N6), and the radio lacks power and bass. Replacing the transformer with a correct type greatly improved sound quality and power.

Be sure that all capacitors have no leakage. Leaky bypass capacitors, can, of course, cause distortion in the audio, but leaky capacitors in the main tone circuit, and all of the other tone circuits (fixed--non-adjustable), will pull down the bass and the overall gain. Leakage here would be the same as placing a resistor across the audio circuit at that point. Decreasing resistance changes the time constant of the capacitors in the circuit and makes it more difficult for them to retain charges for longer periods of time, such as would be the case with lower frequency tones. This is why increasing the resistances I mentioned earlier increases bass response (as well as gain, due to the over-all shunting effect of the resistor).

T.

Thanks for all the infos.
I have now plenty of ideas to checkout and improve my audio circuit.

And yes the paper caps all been replaced during restoration.
But i didn't replace the mica caps in the rest of the circuitry. Do the mica caps sometime can leak too ?

And yes the audio transformer is not the original.
I just forgot to mention it...I thought my replacement was good.
I did replaced the audio transformer with the only one i had and which is a bit smaller then the original.

When i tested the audio after the restoration, i pulled out the two 6AC5 (one at the time) to see if audio remains just to test both side of the push-pull. One side of the push pull wasn't working. So the push-pull was Half working.

I found an open winding in one side of the primary audio transformer.
The new transformer i installed is a Hammond that has a DC resistance of 230 ohms (115 ohms-Tap-115 ohms)
It is the half of what you suggest!

Now it's too late to measure the DC resistance of the original transformer. I throw it to garbage last week.

But i have an other set that i restored and which has a push-pull audio transformer on the speaker. I might try to test my Crosley with that one. It's not the same speaker socket, but with jumpers i will try that before i look for a new audio transformer.

Thak again to all
Vianney

:I would be sure that the output transformer was the original one, and that someone didn't install an incorrect replacement. The DC resistance should be in the neighborhood of 500 ohms. Someone put an incorrect replacement output transformer in a Silvertone radio I own that uses a 6B5 (6N6), and the radio lacks power and bass. Replacing the transformer with a correct type greatly improved sound quality and power.
:
:Be sure that all capacitors have no leakage. Leaky bypass capacitors, can, of course, cause distortion in the audio, but leaky capacitors in the main tone circuit, and all of the other tone circuits (fixed--non-adjustable), will pull down the bass and the overall gain. Leakage here would be the same as placing a resistor across the audio circuit at that point. Decreasing resistance changes the time constant of the capacitors in the circuit and makes it more difficult for them to retain charges for longer periods of time, such as would be the case with lower frequency tones. This is why increasing the resistances I mentioned earlier increases bass response (as well as gain, due to the over-all shunting effect of the resistor).
:
:T.
:

:Thanks for all the infos.
:I have now plenty of ideas to checkout and improve my audio circuit.
:
:And yes the paper caps all been replaced during restoration.
:But i didn't replace the mica caps in the rest of the circuitry. Do the mica caps sometime can leak too ?
:
:And yes the audio transformer is not the original.
:I just forgot to mention it...I thought my replacement was good.
:I did replaced the audio transformer with the only one i had and which is a bit smaller then the original.
:
:When i tested the audio after the restoration, i pulled out the two 6AC5 (one at the time) to see if audio remains just to test both side of the push-pull. One side of the push pull wasn't working. So the push-pull was Half working.
:
:I found an open winding in one side of the primary audio transformer.
:The new transformer i installed is a Hammond that has a DC resistance of 230 ohms (115 ohms-Tap-115 ohms)
:It is the half of what you suggest!
:
:Now it's too late to measure the DC resistance of the original transformer. I throw it to garbage last week.
:
:But i have an other set that i restored and which has a push-pull audio transformer on the speaker. I might try to test my Crosley with that one. It's not the same speaker socket, but with jumpers i will try that before i look for a new audio transformer.
:
:Thak again to all
:Vianney
:
:
:
::I would be sure that the output transformer was the original one, and that someone didn't install an incorrect replacement. The DC resistance should be in the neighborhood of 500 ohms. Someone put an incorrect replacement output transformer in a Silvertone radio I own that uses a 6B5 (6N6), and the radio lacks power and bass. Replacing the transformer with a correct type greatly improved sound quality and power.
::
::Be sure that all capacitors have no leakage. Leaky bypass capacitors, can, of course, cause distortion in the audio, but leaky capacitors in the main tone circuit, and all of the other tone circuits (fixed--non-adjustable), will pull down the bass and the overall gain. Leakage here would be the same as placing a resistor across the audio circuit at that point. Decreasing resistance changes the time constant of the capacitors in the circuit and makes it more difficult for them to retain charges for longer periods of time, such as would be the case with lower frequency tones. This is why increasing the resistances I mentioned earlier increases bass response (as well as gain, due to the over-all shunting effect of the resistor).
::
::T.
::
:
:
There's a good tutorial on output transformers and impedance at www.radioremembered.org. It shows a single ended 6AC5 ideal plate load impedance of 4Kohms so a push-pull config. would probably be around 6-8Kohms. Z is different than the DC resistance of the transformer windings but gives one an idea of the approximate resistance needed. My guess is at least part of your problem is that the primary windings of your replacement transformer should have higher DC resistance for proper impedance matching. You can go through the tutorial and measure the resistances of the speaker VC ect. and get an ideal what the Z and turns ratio should be close to.
From this you can determine what transformer you need. Good luck, John

I think that this is indeed key to your problem. Too low a primary impedance will significantly reduce bass and power.

Regarding mica capacitors, they rarely short or become leaky. Sometimes, especially in non-RF critical circuits (such as the audio circuit), they will look like mica capacitors (bakelite enclosure), but will contain a paper unit. Those become leaky and short and arc. If you've ever had a radio with crusty sounds that come and go, you'd likely find it to be caused by a .001 MFD mica-looking capacitor on the plate of the 1st audio tube. The crusty crackling sounds come from arcing within.

Otherwise the real mica capacitors are quite reliable, and mica is usually what's used in the RF section. Also, capacitors there won't likely affect your audio.

...Back to the audio section, I'd first replace the output transformer with one close to what's appropriate for those tubes, and then, if the audio still needs tailoring, adjust resistances as I mentioned. Usually radios are well designed from the factory, especially when designed by major manufacturers like Crosley.

I personally like to tailor the audio in small ways on all of my radios, such as adding negative feedback, and otherwise adjusting the tone to remove peaks and valleys I find objectionable, but other than that they typically sound fairly good as-is. Once I heard an amplifier (that I built from a late 30s Magnavox schematic) with negative feedback (coming from a 2A3!), I could never go back to audio without it, so I'm kind of spoiled in that way.

T.

115 ohms DC resistance each side of center tap seems low. 6AC5's in push-pull should feed an impedance of 10,000 ohms, plate to plate.

Would expect DC resistance to be more like 500 ohms, center tap to each side. Looking through schematics your resistance readings are more like those for 50L6's where 2000 ohm impedance is required.

Norm

:I think that this is indeed key to your problem. Too low a primary impedance will significantly reduce bass and power.
:
:Regarding mica capacitors, they rarely short or become leaky. Sometimes, especially in non-RF critical circuits (such as the audio circuit), they will look like mica capacitors (bakelite enclosure), but will contain a paper unit. Those become leaky and short and arc. If you've ever had a radio with crusty sounds that come and go, you'd likely find it to be caused by a .001 MFD mica-looking capacitor on the plate of the 1st audio tube. The crusty crackling sounds come from arcing within.
:
:Otherwise the real mica capacitors are quite reliable, and mica is usually what's used in the RF section. Also, capacitors there won't likely affect your audio.
:
:...Back to the audio section, I'd first replace the output transformer with one close to what's appropriate for those tubes, and then, if the audio still needs tailoring, adjust resistances as I mentioned. Usually radios are well designed from the factory, especially when designed by major manufacturers like Crosley.
:
:I personally like to tailor the audio in small ways on all of my radios, such as adding negative feedback, and otherwise adjusting the tone to remove peaks and valleys I find objectionable, but other than that they typically sound fairly good as-is. Once I heard an amplifier (that I built from a late 30s Magnavox schematic) with negative feedback (coming from a 2A3!), I could never go back to audio without it, so I'm kind of spoiled in that way.
:
:T.
:

:Hi,
:
:I'm restoring a Crosley 60 which has 5 tubes to do the audio output circuit !!!!
:A pair of 6J5-6AC5 and another pair of 6J5-6AC5 as push-pull amplifier, and a third 6J5 used as a phase inverter.
:
:I'm not impress at all. The sound is not bad, but there is no bass, and not as much power as my other radios that has only a single 6F6 as audio output.
:
:I have another Crosley that has a pair of pentodes 41M driven by a 6J5, and the audio is very powerfull.
:
:So pentodes are better than triodes in audio amplifiers ?
:
:Well is it normal that a push-pull of triodes like 6AC5 doens't drive power as much as a single pentode or push-pull pentodes ?
:

First, the 6AC5 and the 6J5 are NOT the same tube; they may have the same basing but they are not interchangeable. If you still don't have enough bass, I would check the speaker, cathode circuit of the push-pulls and the output transformer.
Johnnysan-

Hi

Circuits are not the same as to gain. The driver tube, 6J5, will usually be wired as a cathode follower. This tube gives no voltage gain. 6AC5 is a high gain triode usually direct coupled to the driver but it doesn't have as much gain as a pentode, 41/42.

Might be able to increase coupling capacity to the driver to improve low frequency response. Low frequency response may be limited by the speaker?

Norm

::Hi,
::
::I'm restoring a Crosley 60 which has 5 tubes to do the audio output circuit !!!!
::A pair of 6J5-6AC5 and another pair of 6J5-6AC5 as push-pull amplifier, and a third 6J5 used as a phase inverter.
::
::I'm not impress at all. The sound is not bad, but there is no bass, and not as much power as my other radios that has only a single 6F6 as audio output.
::
::I have another Crosley that has a pair of pentodes 41M driven by a 6J5, and the audio is very powerfull.
::
::So pentodes are better than triodes in audio amplifiers ?
::
::Well is it normal that a push-pull of triodes like 6AC5 doens't drive power as much as a single pentode or push-pull pentodes ?
::
:
:
:First, the 6AC5 and the 6J5 are NOT the same tube; they may have the same basing but they are not interchangeable. If you still don't have enough bass, I would check the speaker, cathode circuit of the push-pulls and the output transformer.
:Johnnysan-
:
:
:
:

Agree with all

Not to state the obvious, but are the tubes in the correct sockets? Please check if you can. Stupider things have got past me more than once.

:Hi
:
: Circuits are not the same as to gain. The driver tube, 6J5, will usually be wired as a cathode follower. This tube gives no voltage gain. 6AC5 is a high gain triode usually direct coupled to the driver but it doesn't have as much gain as a pentode, 41/42.
:
: Might be able to increase coupling capacity to the driver to improve low frequency response. Low frequency response may be limited by the speaker?
:
:Norm
:
:::Hi,
:::
:::I'm restoring a Crosley 60 which has 5 tubes to do the audio output circuit !!!!
:::A pair of 6J5-6AC5 and another pair of 6J5-6AC5 as push-pull amplifier, and a third 6J5 used as a phase inverter.
:::
:::I'm not impress at all. The sound is not bad, but there is no bass, and not as much power as my other radios that has only a single 6F6 as audio output.
:::
:::I have another Crosley that has a pair of pentodes 41M driven by a 6J5, and the audio is very powerfull.
:::
:::So pentodes are better than triodes in audio amplifiers ?
:::
:::Well is it normal that a push-pull of triodes like 6AC5 doens't drive power as much as a single pentode or push-pull pentodes ?
:::
::
::
::First, the 6AC5 and the 6J5 are NOT the same tube; they may have the same basing but they are not interchangeable. If you still don't have enough bass, I would check the speaker, cathode circuit of the push-pulls and the output transformer.
::Johnnysan-
::
::
::
::
:
:

Interesting point. 6AC5 and 6J5 have the same basing so radio would somewhat work even if mixed up.

Norm

:Agree with all
:
:Not to state the obvious, but are the tubes in the correct sockets? Please check if you can. Stupider things have got past me more than once.
:
::Hi
::
:: Circuits are not the same as to gain. The driver tube, 6J5, will usually be wired as a cathode follower. This tube gives no voltage gain. 6AC5 is a high gain triode usually direct coupled to the driver but it doesn't have as much gain as a pentode, 41/42.
::
:: Might be able to increase coupling capacity to the driver to improve low frequency response. Low frequency response may be limited by the speaker?
::
::Norm
::
::::Hi,
::::
::::I'm restoring a Crosley 60 which has 5 tubes to do the audio output circuit !!!!
::::A pair of 6J5-6AC5 and another pair of 6J5-6AC5 as push-pull amplifier, and a third 6J5 used as a phase inverter.
::::
::::I'm not impress at all. The sound is not bad, but there is no bass, and not as much power as my other radios that has only a single 6F6 as audio output.
::::
::::I have another Crosley that has a pair of pentodes 41M driven by a 6J5, and the audio is very powerfull.
::::
::::So pentodes are better than triodes in audio amplifiers ?
::::
::::Well is it normal that a push-pull of triodes like 6AC5 doens't drive power as much as a single pentode or push-pull pentodes ?
::::
:::
:::
:::First, the 6AC5 and the 6J5 are NOT the same tube; they may have the same basing but they are not interchangeable. If you still don't have enough bass, I would check the speaker, cathode circuit of the push-pulls and the output transformer.
:::Johnnysan-
:::
:::
:::
:::
::
::
:
:

Hi,

Yes sure ! 6AC5 are in the right sockets and driving the audio transformer.

::Not to state the obvious, but are the tubes in the correct sockets? Please check if you can. Stupider things have got past me more than once.
:

I strongly believe that the output transformer is your problem. Find one for an amplifier that uses push-pull 6F6/6K6/41/42, and that will be close enough, though even that isn't quite as high in impedance as it should be.

T.