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7868 amp sounds good but low volume

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I've rewired my Precision tube amp to use a 12AX7 preamp tube and it now sounds pretty good.

The volume, however, is nowhere near the 32 watts those 7868 tubes are rated for. I'm using a small 16-ohm speaker from a Marshall mini-stack cab.

Attached is the new schematic with voltages.
The Bogen CHB-35A is very similar.

Apparently the 6C4 cathodyne phase inverter doesn't like to be pushed too hard. The distortion is very unpleasant. The 6c4 did test good on my Sencore, but I'm wondering if the problem is the phase inverter.

I did swap output transformers without any change.

I'm out of ideas.

Any suggestions would be greatly appreciated!


  • precision_1.pdf
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>> You seem to be describing class C

The amp is grounded cathode, which I believe would be class B or AB.
With class C, the tube would be completely off at idle.
Is this correct?

Maybe I should check the 7868's on my Sencore. ... They're fine.

Are those 5 ohm resistors in front of the bridge rectifier necessary?
If not, I could take them out and use them to check the idle current of the 7868s.

>> Scratchy sounds can come from dirty pins and failing tubes
The 6C4 is the only old tube. It tested fine on the Sencore.
It's an uncommon tube (7-pin).
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Stick a 1 ohm resistor in series with each 7868 cathode to ground.. I am wondering whether the output tubes are running near or beyond cut-off.

Here 1mV would equal 1mA, 10mV 10mA, and so on.. I would expect something in the range of 30mA - 40mA would be reasonable.

I am wondering about the 2.5K resistor value feeding the screens of those 7868 - seems like a rather high value to me.. (I could be wrong..) If the screens draw appreciable current the voltage at this point will drop a lot.

The 6C4 is not all that uncommon, available from Antique and eBay..

Measure the voltages in various different parts of the circuit and post them here, that ought to give us a good idea of whats going on.
7868 bias

I picked up some 1-ohm resistors from Radio Shack to set the bias.

Here's a quote on the bias for 7868's

The RCA tube manual shows that the grid voltage on the 7591/7868 should be around -21 volts. Fisher uses around -15 or -16 volts on the grids to get more power output. NOS 7591's are hard to come by and the new variety won't last very long with a lower grid voltage.

The 7868's maximum plate dissipation is around 19 watts so you want to bias at around 65 percent of that @ class AB1. So that would be a bias current of around 32 mA @ a plate voltage of 393 volts.

The plates have 424 volts.

Initially I measured 38mV bias on each tube. The grids were at -17 volts.

By ear, the best setting was -21 grid volts with 18.6 and 18.2 mV bias.

It was surprisingly sensitive. A little change either way resulted in nasty distortion.

The overall volume did increase, but it is still extremely low.

Perhaps there's not enough gain in the preamp. The 7868's need 25 volts to drive them, which is similar to an EL84.

I have 2 12AX7 gain stages of about 100 (mu) with a tone stack in the middle, followed by the cathodyne phase inverter with unity gain. The original had one more, which I think is necessary.

I could take change the 6C4 cathodyne to a 12AX7 long-tail-pair, which would add gain and use a tube I can easily get locally.

Anybody have opinions as to phase inverters: long-tail-pair vs. cathodyne?

It's a guitar amp, and I've heard that LTP distorts better, although the tweed deluxe uses cathodyne, and that's a legendary tone machine.

The 6C4 is a 7 pin mini type that, for all intents and purposes, is 1/2 a 12AU7. The 6J6 is a 7 pin mini high gm/low Rp twin triode, with a common connection for both cathodes. The 6J6 IS your LTP splitter.

You may have to add some extra filament power, as the 6C4 draws 150 mA., while the 6J6 draws 450 mA.

BTW, did the EMail I sent you a few days ago arrive?
>> BTW, did the EMail I sent you a few days ago arrive?

Yes. I didn't have time to respond.
I've been pulling my hair trying to figure out what was wrong with this amp.

The amp originally had a second 6EU7, so there's a spot for another 12AX7, plus enough filament power.

At this point it's a design issue.

I've made several guitar amps with a 12AX7 preamp and 12AU7 long tail pair, which might be nice in this amp.

I'm considering putting it into a new chassis and making a tweed combo for guitar.
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