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6SN7 & 6V6 Push Pull - increase gain

laosei

Member
2013-02-25 11:51 am
Hi, this is my first post on DIYAUDIO. I recently purchased a 6V6 push pull power amplifier which has 6SN7 as the pre-stage. I would like to increase the gain but do not want to void the warranty. Does anyone know of tube substitutes that can increase the gain?

Thank you in advance for any assistance!
 
ummmmm, why? What other changes are you intending to make at the same time? Or are you just looking for some turbocharge result from a plug-in substitution?

Your question has two problems - it assumes a recommendation can be made on information so limited that its impossible; and it is based on a belief that "stuff" can be swapped about to acheive significant results with no other considerations.

The short answer is "no". The long answer is "perhaps, depending on a load of other variables, not the least your ability to solder..."
 

laosei

Member
2013-02-25 11:51 am
I need more gain because I removed a preamp from the chain. I no longer use two sources, instead just a usb -> spdif converter (volume is controlled in the digital domain), but the output of my dac is not enough to drive the Push Pull power amp to sufficient volumes.

I don't have a copy of the circuit, but I can solder to the point of replacing resistors/capacitors, etc. What do we need to change to accommodate the 6SL7?
 
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laosei

Member
2013-02-25 11:51 am
Ok sounds like the simplest plan. I think there definitely needs to be a gain stage between my dac and power amp. Is it wiser to build a gain stage that has no volume attenuation and control the volume in the digital domain? My DAC is non-oversampling and has two 10 ohm resistors at the output, hence the low level output - so presumably it is better for the DAC chip to have a constant load after these resistors rather than varying the load (which is what a preamp does)?
 
here's the problem - the circuit (if competently designed) is already optimised for all of the current components. Change any one component and you are required to adjust the rest of the circuit.

You can easily say "replace the sn7 with an sl7" but the reality is that the sl7 has 5 times the mu of the sn7, but only one fifth of its current capability, will need the anode and cathode resistors calculated and changed, will in all likelihood overdrive the 6V6's so a change to feedback ratios will be required, may not have the current capability to handle transients so could require an intermediate stage etc etc etc...

Its like asking can I put a 750cfm holley double pumper on my Datsun 1600 - its a bit slow atm.

Yeah, you can - no it won't fix the issue.

Put the pre-amp back in. Its there for a reason.
 
Ok sounds like the simplest plan. I think there definitely needs to be a gain stage between my dac and power amp. Is it wiser to build a gain stage that has no volume attenuation and control the volume in the digital domain? My DAC is non-oversampling and has two 10 ohm resistors at the output, hence the low level output - so presumably it is better for the DAC chip to have a constant load after these resistors rather than varying the load (which is what a preamp does)?

Any competent preamp won't load the DAC appreciably and certainly won't alter the load based on the pot position
 

Cassiel

Disabled Account
2004-09-30 3:53 pm
Madrid
The problem here is that most likely the bias resistor value is too low for the 6SL7. That means too much current through the 6SL7 - a low current tube. If this is the case the 6SL7 won't live long. Some designers bias the 6SN7 with a 1K resistor and that would be OK for the 6SL7, even if the plate resistor it's not optimal. Yes, a little more distortion but it's not a big deal, most people wouldn't notice it because it's inside the feedback loop. In short, the swap can work but most likely it won't. We need to know the schematic to be sure.


Ok sounds like the simplest plan.

No. The simplest plan is to change two resistors. Having to add a preamp because you can't change the value of two resistor seems to me........well.....not wise.
 

Cassiel

Disabled Account
2004-09-30 3:53 pm
Madrid
OK, this a DIY site and sometimes I forget some people don't DIY. Actually I was assuming the amp has another tube for the phase splitting duties, if this is not the case my words weren't that wise.... yes, going back to the preamp would be a better idea. Also, if you don't know what you're doing.... getting electrocuted is not that wise either.
 

lordearl

Member
2007-06-14 1:42 pm
He has stated he is comfortable in 'soldering resistors'. I'd be inclined to side with Cassiel and suggest getting the schematic, THEN seeing what can be done. Either that or adding a small opamp based gain stage after the DAC to get it to the standard 2vrms, just to test whether that drives the Push Pull amp a little better. Plenty of options, all of which involve a bit more work than just replacing the preamp (or getting a better preamp).
 
Put the preamp back in.[4]
By the way, pulling a perfectly working preamp and then having to build "something" to replace it .... which by the way would be another "preamp" ;) sounds, well .... weird.
Very much doubt modding the existing tube amp, by tube or parts substitution or both will provide 5X more gain than you have now.
And that, even if possible, will at best modify its sound and at worst be a mess.

Modding your digital player to provide 5X output signal?
See comment above. :(