Hot Rod Deville- New wattage?

[Chuck H]

Lots of speculation. Doesn't even take into account the effect of global NFB.
In case of doubt I prefer to measure.

Agree. But I don't have the amp on my bench. So...

I ran a simulation. Using the test voltages on the HRD schematic to align an amp model that is similar enough. I don't have a 12au7 model but I split the plate loads on the PI for a 5:1 ratio which I assume would be similar to the different amplification factor. The output dropped considerably so I started bumping input voltage until the wave form started compressing. This was at around 36V peak to peak out of the PI. Not enough to drive the output to full power.

[Helmholtz]

Agree. But I don't have the amp on my bench.

But the OP has.

I split the plate loads on the PI for a 5:1 ratio which I assume would be similar to the different amplification factor.

The amplification factor µ of a triode is a theoretical value which does not linearly translate to actual gain.
LTPI gain ratio between ECC83 and ECC82 should be close to 3.
Nevertheless you might have a valid point here and the resulting off-center biasing of the ECC82/12AU7 will make things even worse.

[pdf64]

The table in this post is good for seeing how other valve types work compared to ECC83 in a standard ECC83 common cathode circuit https://www.thegearpage.net/board/in...table.2279173/

[Helmholtz]

I recommend to read the whole TGP thread linked by pdf64 above.
There's some excellent explanation why a 12AU7 performs poorly in a 12AX7 circuit, especially regarding headroom/early single sided clipping.

[Helmholtz]

Also highly recommended: R.Kuehnel's calculators on ampbooks.
https://www.ampbooks.com/mobile/ampl...g-tailed-pair/

[Chuck H]

Also highly recommended: R.Kuehnel's calculators on ampbooks.
https://www.ampbooks.com/mobile/ampl...g-tailed-pair/

I access that site sometimes and endorse this.

[nevetslab]

I have slightly revised a lot of the Hot Rod series amps leaving the input tube stage w/ 12AX7 in place but change the second tube V2 to a 12AY7 and revised the cathode bypass circuits in both the Drive and MORE DRIVE stages. Dropped the rather excessive gain boost that was typically more than 10dB different to a more tame 5 to 6dB boost.

I know I’ve posted the details a few years ago. Will dig thru my archives and come back with the link to clarify.

I am on my iPhone and discovered I can’t access my 1400+ subscriptions that lists all of the threads I’ve contributed on/to so can’t find that one thread I had posted with the results showing both the gain difference as well as the measured acoustical output difference. Still kept the extra boost in MORE DRIVE mode, just tamed it down some which most players at CenterStaging LLC found more pleasing.

I found the thread t had posted on 11/09/2017 using the search words DRIVE/MORE DRIVE MOD. That took me to the Fender Hot Rod Deluxe/Deville DRIVE/MORE DRIVE MOD which ran a good 3-4 pages of data and photos. As I am on my iPhone 15, I can’t get the thread address, but had no trouble pulling up the thread I had begun with lots of freq response and acoustic measurement data that I had never seen before.

Enjoy……nevetslab

[g1]

I found the thread t had posted on 11/09/2017 using the search words DRIVE/MORE DRIVE MOD. That took me to the Fender Hot Rod Deluxe/Deville DRIVE/MORE DRIVE MOD which ran a good 3-4 pages of data and photos. As I am on my iPhone 15, I can’t get the thread address, but had no trouble pulling up the thread I had begun with lots of freq response and acoustic measurement data that I had never seen before.

Here is the link: https://music-electronics-forum.com/...-repair/46526-

[Iplayloud]

I'm generally not in favor of swapping preamp tubes with other types, without re-biasing for the same plate voltage that is.
There's one useful exception: Using a 12AY7 instead of a 12AX7 as input tube. This will lower gain by 6dB per section.

I think the 5751 is also fully acceptable as well right?

Lots of speculation. Doesn't even take into account the effect of global NFB.
In case of doubt I prefer to measure.

Just to be sure, you mean using another PI tube like a 12au7 will affect the NFB ?

Also I thought for a 12ax7 circuit only the 5751 and 12ay7 were acceptable, and for 12at7 circuits only a 12au7 was acceptable as a lower gain replacement. However looking at the linked chart, the 12at7 and 12ay7 have a very similar plate current, so the concern may not be founded?
I suppose going from a 12ax7 (1.04 Amp) to a 12au7 (2.15 Amps) would be a problem however.

[Helmholtz]

I think the 5751 is also fully acceptable as well right?

Yes, but will lower gain by only 2dB.

Just to be sure, you mean using another PI tube like a 12au7 will affect the NFB ?

The PI tube is within the NFB loop and belongs to the power amp.
The PI gain with a 12AU7 is 10dB lower than with a 12AX7, so much less loop gain and effective NFB.

Also I thought for a 12ax7 circuit only the 5751 and 12ay7 were acceptable, and for 12at7 circuits only a 12au7 was acceptable as a lower gain replacement. However looking at the linked chart, the 12at7 and 12ay7 have a very similar plate current, so the concern may not be founded?
I suppose going from a 12ax7 (1.04 Amp) to a 12au7 (2.15 Amps) would be a problem however.

Yes, I was surprized to see that the 12AY7 and the 12AT7 perform quite similar when used as input tube.
Another surprizing result was that Kuehnel's PI calculator actually shows a slight increase in gain when substituting a 12AT7 for a 12AX7 in a typical LTPI circuit, while the 12AY7 and the 5751 give significantly lower gains.
Means that the circuit makes a lot of difference.

[mozz]

12AT7 can be a lot more microphonic than a 12ax7. Look for a 6201 or 12at7wa with the triple micas.

[pdf64]

...
Another surprizing result was that Kuehnel's PI calculator actually shows a slight increase in gain when substituting a 12AT7 for a 12AX7 in a typical LTPI circuit, ...

That doesn't seem correct to me, what do you think?

I question whether the valve type models are correct.

[Helmholtz]

That doesn't seem correct to me, what do you think?
I question whether the valve type models are correct.

Yes, I'm wondering if his 12AT7 model is flawed.
According to Merlin LTPI gain is half the gain of a fully bypassed single gain stage without a load in both cases.
This rule seems to work ok with the calculator results for the other tube types (LTPI gain is a bit lower because of lower load resistor used compared to HBP's table).

[Chuck H]

I'm out of my depth now, but... Is it possible the at7 handles any following input impedance better and that is the reason for the higher gain?

[Helmholtz]

I'm out of my depth now, but... Is it possible the at7 handles any following input impedance better and that is the reason for the higher gain?

As said, I think his 12AT7 tube model is flawed.
An LTPI gain of 26 seems way too high, I would have expected something like 17 with the 12AT7.
The difference cannot be explained by the low output impedance.
The results for other types (12AX7, 12AU7, 12AY7, 5751) look ok.