Changing phase inverter valve

bandmaster188

Just been reading an amp book and there was mention to fender changing the PI valve (12ax7 to 12at7)in amps like the super reverb and bandmaster so that there is less gain going to the output tubes for more clean headroom. Was just wondering if its safe to try a 12ax7 in the PI position for a bit more breakup on my bandmaster reverb? Or does it affect biasing or melt stuff. What do ya reckon?

ecc83

The Bandmaster 6G7-A does in fact use a 12AX7 (well, a 7025 is shown) but yes, most models I have schematics for show a 12AT7. 

The cathode resistances shown for the 7025 are much lower than for the X7 so, I don't think you will damage anything but it might not sound good!

"IC" and "Jay" will I am sure have the full dope.

Dave.

ICBM

Yes, it's safe. Fender originally used 12AX7s, but swapped to 12AT7s to improve clean headroom, when that was the goal in guitar amps. The biasing is slightly different, but not enough to cause any harm. It sounds pretty good in my experience - a bit warmer and 'hairier' rather than obviously earlier breakup.

For what it's worth I really like the opposite swap in amps which are normally too grainy and midrangy with a 12AX7 in the PI, like many Mesas. A 12AT7 opens out the sound and gives more depth and clarity.

bandmaster188

cheers fellas's think i'll give it a go. the 73 bandmaster tfl 5005d i've got definately shows a 12at7 on the valve map. will let you know how it goes!

 

jpfamps

It's safe to try, but you don't actually get any more gain due to the design of the circuit in that amp.

You will also get slightly worse balance, although I doubt this will account for any changes in performance.

In the later SF amps with 47k anode load resistors in the PI you actually get less gain with a 12AX7 than a 12AT7.

If you prefer the sound that's fine though, but any differences won't be due to increased gain.

ecc83

jpfamps said:

It's safe to try, but you don't actually get any more gain due to the design of the circuit in that amp.

You will also get slightly worse balance, although I doubt this will account for any changes in performance.

In the later SF amps with 47k anode load resistors in the PI you actually get less gain with a 12AX7 than a 12AT7.

If you prefer the sound that's fine though, but any differences won't be due to increased gain.

Aha! Just spotted the NFB loop!

Dave.

jpfamps

ecc83 said:

jpfamps said:

It's safe to try, but you don't actually get any more gain due to the design of the circuit in that amp.

You will also get slightly worse balance, although I doubt this will account for any changes in performance.

In the later SF amps with 47k anode load resistors in the PI you actually get less gain with a 12AX7 than a 12AT7.

If you prefer the sound that's fine though, but any differences won't be due to increased gain.

Aha! Just spotted the NFB loop!

Dave.

Nothing to do with the NFB loop, it's an inherent property of the LTP.

ecc83

jpfamps said:

ecc83 said:

jpfamps said:

It's safe to try, but you don't actually get any more gain due to the design of the circuit in that amp.

You will also get slightly worse balance, although I doubt this will account for any changes in performance.

In the later SF amps with 47k anode load resistors in the PI you actually get less gain with a 12AX7 than a 12AT7.

If you prefer the sound that's fine though, but any differences won't be due to increased gain.

Aha! Just spotted the NFB loop!

Dave.

Nothing to do with the NFB loop, it's an inherent property of the LTP.

So, NFB does not tend to make overall gain far less dependent upon individual stage gains?

Dave.

bandmaster188

Calm down fella's >:/ What's LTP?

Misterg

LTP = Long Tailed Pair

i.e. the specific type of phase inverter in your amp.

I'm with JPF - you can get less gain in a phase inverter with a 12AX7 / ECC83 than a 12AT7/ECC81, depending on the details of the circuit.

AFAIK, you can't hurt anything by trying it (maybe not always true going in the other direction).

ecc83

"I'm with JPF - you can get less gain in a phase inverter with a 12AX7 / ECC83 than a 12AT7/ECC81, depending on the details of the circuit."

Did not say you couldn't.

Dave.

Misterg

Never said you did  :P

Misterg

Double post....

Misterg

How did that happen, then ???  @-)

bandmaster188

Misterg said:

LTP = Long Tailed Pair

i.e. the specific type of phase inverter in your amp.


Ah yes. I just read about that in the book. Clearly I need to pay more attention!!!

jpfamps

ecc83 said:

jpfamps said:

ecc83 said:

jpfamps said:

It's safe to try, but you don't actually get any more gain due to the design of the circuit in that amp.

You will also get slightly worse balance, although I doubt this will account for any changes in performance.

In the later SF amps with 47k anode load resistors in the PI you actually get less gain with a 12AX7 than a 12AT7.

If you prefer the sound that's fine though, but any differences won't be due to increased gain.

Aha! Just spotted the NFB loop!

Dave.

Nothing to do with the NFB loop, it's an inherent property of the LTP.

So, NFB does not tend to make overall gain far less dependent upon individual stage gains?

Dave.

I think you are confusing the effects of global NFB with the operation of the LTP.

As you correctly state, applying global negative feedback around the power amp will linearise the output and the gain will tend to be determined by the amount of feedback applied. This later assumption assumes that the gain of the amp is very high and as is the feedback factor. The open loop gain of guitar amps is quite low as is the amount of negative feedback (assuming you are using negative feedback; so amps don't), so the assertion that the gain of the amp is set by the amount of feedback will be more of an approximation for a guitar amp (unlike say a SS amp).

The gain from the LTP circuit used in Fender amps will be the same with either a 12AT7 or a 12AX7 even without negative feedback from the output stage.

If you don't want to plough through the maths, here is a useful calculator from Richard Keuhnel's website (by the way his books are excellent) where you can change the circuit values + valves in the LTP and the gain of each side is calculated for you. NB, the calculator is for the LTP with NO negative feedback applied from the amp's output.

http://www.ampbooks.com/home/amplifier-calculators/long-tailed-pair/

ecc83

I am obliged for the PI calculator link and have run a the AT7 and AX7* through it with a couple of different values of anode load. 

There is barely  more a couple of dBs in it. 

However I have never disputed that fact and I cannot  recall what the point of this discussion is or was? 

*The 12AU7 is well down on gain.

Dave.

ICBM

ecc83 said:

I cannot  recall what the point of this discussion is or was?

Whether it's safe and/or whether it changes the sound.

Both, in my opinion - but as I said earlier, it's more that the 12AX7 is 'warmer' or 'hairier' sounding and that the 12AT7 is 'deeper' or 'clearer' sounding rather than any change in gain or breakup.

As jpfamps suggested, this could actually be to do with the change in balance rather than anything to do with the gain - a less balanced PI will produce more harmonic distortion, which may be what I hear with the 12AX7.

ecc83 said:

I am obliged for the PI calculator link and have run a the AT7 and AX7* through it with a couple of different values of anode load. 

There is barely  more a couple of dBs in it. 

*The 12AU7 is well down on gain.

And is actually a useful substitute in Marshall valve bass models, when you want to use them for bass.