Ampeg 835 question

[Pedro Vecino]

Hello everyone. About 25 years ago I had the opportunity to work with an Ampeg 835, a precursor in some ways to other models such as the Portaflex. It created a very good impression on me (for guitar).
I'm going to build one but with a fixed bias circuit and some adaptations. One of them is a master volume after the phase splitter. I wonder if the preamp, with a 6CG7 (similar in gain to a 12AU7) and a 6SL7 (similar in gain to a 5751) as a phase splitter in what appears to be a variant of a classic paraphase circuit would allow me to obtain some degree of distortion with the master volume or if it would be very limited. I never used that amp at high volume.
Is it possible to estimate approximately what can happen? In the case of being very limited, I could consider using another 6SL7 in the preamp and accommodate the circuit in some way.
I have recomposed the original schematic with the values on the components to be able to interpret it since the original has the latter on another page and it is very difficult to interpret it that way.
Any ideas? Thanks!

[Pedro Vecino]

Well, in the end I didn't want to risk being short on overdrive (6CG7 triodes with bypass cathode capacitors) and I used the B15n preamp with 6SL7 as a reference. This by itself has some limitations in that aspect but with a bypass cathode capacitor added to the second triode itīs already positioned to have good overdrive level together with the master volume.
I also didn't use the OC3 tube applied as zener to the screen grids. I thought about making a selectable circuit (direct/0C3) together with an automated bias circuit (the amp is with fixed bias; I have no interest in cathode bias in this project) but in the end I preferred to use a double bias circuit, independent for each tube.
With everything already distributed on the chassis I will probably use a smaller 0B2 one day to experiment. Itīs about subtracting 108 volts from the screen grids and see what happens.
The amp has two different channels (Ampeg and Fender style) and what I can anticipate is the need for heaters elevation and manual adjustment with a potentiometer. The center tap of the transformer worked fine with the first channel but not with the second.
I used a 500 Ohm/5 watt CTS potentiometer and the adjustment was so close to one of the extremes that I had to use a 220 Ohm resistor in parallel on one side so that the final adjustment would be better centered.

I have a question regarding the bias of the 6SL7 on the Fender style channel. I have used the same values (100K/1K5) as in the classic Fender amps with 12AX7. Would it be convenient to rearrange these values using this tube? If so, in which direction? I see that the trend in its use is to use higher value plate resistors...

Thank you very much everyone.

[catalin gramada]

As far you have heaters noise concerns just two suggestions you may try: First, get heaters reference at the first tube in the chain (meant two 100 ohms resistors soldered directly on the first input preamp socket and grounded directly at the input jack ground)- of course heaters winding center tap will be suspended. Elevated heaters is also an option but try both with positive and negative voltage. Sometimes negative voltage reference work better...You may consider also unbypassed cathode in the first gain stage is more prone to pickup noise from heater filaments than a fully bypassed cathode.

[Helmholtz]

I have a question regarding the bias of the 6SL7 on the Fender style channel. I have used the same values (100K/1K5) as in the classic Fender amps with 12AX7. Would it be convenient to rearrange these values using this tube? If so, in which direction? I see that the trend in its use is to use higher value plate resistors..

No need to change resistor values with the 6SL7.

[Pedro Vecino]

As far you have heaters noise concerns just two suggestions you may try: First, get heaters reference at the first tube in the chain (meant two 100 ohms resistors soldered directly on the first input preamp socket and grounded directly at the input jack ground)- of course heaters winding center tap will be suspended. Elevated heaters is also an option but try both with positive and negative voltage. Sometimes negative voltage reference work better...You may consider also unbypassed cathode in the first gain stage is more prone to pickup noise from heater filaments than a fully bypassed cathode.

Thanks. I had already guessed that I would have to deal with filament noise. This is my first build with octal tubes. The truth is that the amp is as quiet as a dead mouse. I mentioned out of curiosity the need (I imagine it will vary somewhat with different tubes) for such an asymmetrical adjustment on the potentiometer. The wiper is at 68 Ohms on one side and 400 on the other approximately. It caught my attention.

No need to change resistor values with the 6SL7.

Thanks too. The Fender-style channel sounds as expected (very good) and I already guessed that the polarization it has would not be too far off. However, I will experiment with it to calculate other possibilities. I have a ruler and a new printer ink cartridge .