5e3 Preamp tubes

[BluesDude]

In my tweed deluxe clone... I have a 12ax7 in the first preamp stage. Apparently the original has a 12ay7 in the 1st preamp stage and the 2nd preamp stage and PI shared a 12ax7.

If I change the 12ax7 in the 1st preamp stage to a 12ay7, the gain in that stage will drop from around 100 to 40.

I read that this will give me more head room before distortion. Will it affect volume (i.e., loudness) also? Will it reduce the overall amount of distortion or just when the onset of distortion begins?

What should I expect out of the change?

[tubeswell]

In my tweed deluxe clone... I have a 12ax7 in the first preamp stage. Apparently the original has a 12ay7 in the 1st preamp stage and the 2nd preamp stage and PI shared a 12ax7.

If I change the 12ax7 in the 1st preamp stage to a 12ay7, the gain in that stage will drop from around 100 to 40.

I read that this will give me more head room before distortion. Will it affect volume (i.e., loudness) also? Will it reduce the overall amount of distortion or just when the onset of distortion begins?

What should I expect out of the change?

Cleaner and slightly fatter tone. A little bit of a drop in volume but not much and its not really worth getting too concerned about. The amp is still plenty loud enough.

[pdf64]

'If I change the 12ax7 in the 1st preamp stage to a 12ay7, the gain in that stage will drop from around 100 to 40'
The voltage gain of the 1st stage will drop from ~60 to ~35.

'I read that this will give me more head room before distortion. Will it affect volume (i.e., loudness) also? Will it reduce the overall amount of distortion or just when the onset of distortion begins?'
I see all the above as being basically power amp characteristics. Altering the pre-amp gain won't affect them. You'll probably have to turn the volume control a little higher to get the same sound pressure level.

[d95err]

In my tweed deluxe clone... I have a 12ax7 in the first preamp stage. Apparently the original has a 12ay7 in the 1st preamp stage and the 2nd preamp stage and PI shared a 12ax7.

If I change the 12ax7 in the 1st preamp stage to a 12ay7, the gain in that stage will drop from around 100 to 40.

I read that this will give me more head room before distortion. Will it affect volume (i.e., loudness) also? Will it reduce the overall amount of distortion or just when the onset of distortion begins?

What should I expect out of the change?

The amp will start distorting at exactly the same loudness level, regardless of what type the first preamp tube is.

[MWJB]

"The amp will start distorting at exactly the same loudness level, regardless of what type the first preamp tube is. " Not really, if you just wanted the amp's volume to be a wider sweep then you can play with the pot value, or try a split load plate resistor at the first stage. Subbing a low mu tube changes the amplification factor, the plate voltage, frequency response and bias of the tube...the amp will not sound "the same" with a 12AU7 in V1 as it will with a 12AX7 in V1.

The amp will still distort, but distort differently. Smoother, less crunchy & dynamic, the lower the mu.

Indeed, the amp won't make significantly more, or less power outright with preamp tube subs (unless you had some really hot pick ups that fed back real early with a 12AX7, or if you fitted such lo gain tubes that you couldn't drive the power tubes), but there will be changes in tone with different preamp tubes. It's worth a try.

[Bruce / Mission Amps]

MWJB is right on the money.
And, the 12AY7 will hit the coupling cap with about 2.7vac to +3vac of signal with a +100mvac drive on it's grid.
A 12AX7 will be almost twice that.
The 12AY7 is almost dead clean at full output with both volume controls at 12, but is still more then enough to slightly overdrive the 12AX7 "volume controls" recovery stage/driver triode and it will clip the phase inverter signal.

If you want to try an interesting mod.... keep use the 12AX7 and a split load resistor on lug 6.
Try something like a 39K and 56K set and use the 56K to the tube socket.
Change the coupling cap to 47nF or less and play around with it.
Or as some of us have done, use a 1/2 w, 100K trim pot as the anode load and split the signal by connecting the coupling cap to the wiper of the pot.
Now just keep messing with the trim pot to get a sound and output voltage you like.

[echuta13]

I had a Champ type build that I ran with a split-load on the first triode. Worked like a charm (for what I wanted)! I think I ran a 47k/56k split on it if I can recall.

[BluesDude]

Thanks for the idea Bruce. Interesting... If I am interpreting you correctly, it in effect can reduce the 12ax7 gain factor to make it closer to the gain factor of a tube like a 12ay7.

I did a search and found a relatively detailed writeup on that approach. It is here: Split Load Plate Resistors on blueguitar.org

[Bruce / Mission Amps]

Thanks for the idea Bruce. Interesting... If I am interpreting you correctly, it in effect can reduce the 12ax7 gain factor to make it closer to the gain factor of a tube like a 12ay7.

I did a search and found a relatively detailed writeup on that approach. It is here: Split Load Plate Resistors on blueguitar.org

No, not the gain but the delivered output to the next stage is comaprable to the 12AY7 and still have the "correct" load to the 12AX7.... good link, and if I remember correctly, back in 2001 (Steve A will correct me if I'm wrong) when asking similar questions like this in emails, I was the one who suggested he also learn about and try a split load resistor on a preamp stage.
Yes, some of us have been here with tboy many many years...

[BluesDude]

No, not the gain but the delivered output to the next stage is comaprable to the 12AY7 and still have the "correct" load to the 12AX7.... good link, and if I remember correctly, back in 2001 (Steve A will correct me if I'm wrong) when asking similar questions like this in emails, I was the one who suggested he also learn about and try a split load resistor on a preamp stage.
Yes, some of us have been here with tboy many many years...

Thanks for the clarification.

When I found that article I quickly reviewed several other documents and noticed several references to you and your advice on split load and some other amp topics.


It occurs to me that one option might be to continue to use the 12ax7 in the first stage... split-load one triode to produce 12ay7 like characteristics and leave the other triode as is... in the quest of a dirty channel and clean(er) channel (with a little more head room).

Am I on the right track?