Hot Rod Deville- New wattage?

[rockman627]

I tinkered with the tubage in my HRD. I have a 12at7 in V1, a 12ax7 in V2, a 12au7 in V3, and TAD 6L6GCs for output. The sound is great, the volume is manageable and the response of the volume knobs is much better. I prefer pedals for my overdrive but a little bit of drive channel gives me clean with a little hair on it, which I also dig. This worked out so well that I suspect some unknown is going to fry my pcb. Can anything go wrong with this setup?
Also, if three 12ax7s produce 60 watts, how many watts am I getting with the new tubes?

[glebert]

Curious what others say on this since this isn't my area. Only talking about clean signal, it seems like a 12at7, which has a gain of 70 compared to 100 for a 12ax7, would give 70% of the swing for the same input. Since it goes through V1 twice it seems like that would then get you half of the swing (0.7*0.7=0.49). Half of the signal swing will get you 1/4 of the power output because of V^2/R. I thought the PI tube (V3) gain wasn't really a direct impact on the drive strength to the power tubes, but I may be wrong on that.

I've done similar tube rolling on Hot Rod amps, but also found that just plugging into input 2 instead of 1 gave a very similar result since it has a voltage divider that cuts the input to the first gain stage in half.

[Helmholtz]

Available power is determined by the output section.
Preamp tubes typically have more than enough gain to fully drive (and overdrive) the power stage.

[glebert]

Available power is determined by the output section.
Preamp tubes typically have more than enough gain to fully drive the power stage.

While it is technically irrelevant, most players think about "how loud is my amp when I set the Volume at X?" in terms of the output power of their amps. Hot Rods are notorious for having a terrible volume taper where it goes from too quiet to too loud in a very narrow range of adjustment, and for having far too much gain in the Drive channel. Reducing the gain in the preamp helps widen out that usable range a bit, and maybe make the Drive more usable. There are products out there specifically for these amps that add a master volume in the effects loops to try to accomplish the same effect for the volume (but it doesn't help the Drive Channel issue).

[Helmholtz]

While it is technically irrelevant, most players think about "how loud is my amp when I set the Volume at X?" in terms of the output power of their amps. Hot Rods are notorious for having a terrible volume taper where it goes from too quiet to too loud in a very narrow range of adjustment, and for having far too much gain in the Drive channel. Reducing the gain in the preamp helps widen out that usable range a bit, and maybe make the Drive more usable. There are products out that that add a master volume in the effects loops of these amps specifically to try to accomplish the same effect for the volume (but it doesn't help the Drive Channel issue).

Yes, I understood.
But I answered the question about possibly reduced output power when using lower gain preamp tubes.
Try this: Use the clean channel and turn the amp fully up. If it distorts, the gain is enough for full output and more.

[mozz]

I've used a 12at7 for a v3 driver in some fender amps of that series. Measured about a watt more. More internet lore of a V1 12ax7 with gain of 100, most common usage your gain is about 51. Swapping in a 12at7 will lower your gain but then the plate and cathode resistors are not optimized for that tube.

[g1]

As far as the question of whether anything might get burnt, you would have to look at the individual triode stages that have been changed, and consider the difference in bias points for those stages in relation to their associated resistor wattages.
As some of these models are known for resistor issues (especially in PI stage), it might be worth it to up the wattage of those resistors (from 1/2W to 1W). And the larger body size of higher wattage resistors usually also gives the benefit of higher voltage handling.

[Helmholtz]

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.

[rockman627]

That's the info I'm looking for. There's a lot of info on the net saying you can swap preamp tubes, but one person mentioned the different voltage requirements. Thanks for the enlightenment.

[Helmholtz]

If it's only about lowering gain, using split-load plate resistors with original tubes might be a good idea.

[rockman627]

I tried the 12ay7. There was no drive at all in the drive channel. For me 12at7 gave me a fairly good range on the volume control and the right amount of drive on the drive channel. I'll have to look up split load plate resistors, but I don't want to do heavy mods on the hrd pcb.

[rockman627]

In a totally unrelated event, I've been meaning to donate to this site for awhile and was usually sidelined by unexpected expenses (car repair etc). I finally got an opportunity and I did it. Many thanks to you guys for all the help. I'm getting better at this because of this forum. Seriously, just this week I fixed a Mesa LoneStar and an Ampeg SVT4 pro without running to you guys for advice. I got a little big-headed for a day. Again, thanks.

[Chuck H]

I have a hunch you haven't actually altered the amps wattage per se'. With the au7 running as the PI you may have limited power tube signal so that the PI is clipping before the power tubes start to clip. So if your amp is putting out fewer watts it's not beause the different gain structure in the preamp reduced the amps wattage. It's because the different gain structure in the preamp no longer drives the amp to it's full wattage.

[Helmholtz]

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

[DrGonz78]

In a totally unrelated event, I've been meaning to donate to this site for awhile and was usually sidelined by unexpected expenses (car repair etc). I finally got an opportunity and I did it. Many thanks to you guys for all the help. I'm getting better at this because of this forum. Seriously, just this week I fixed a Mesa LoneStar and an Ampeg SVT4 pro without running to you guys for advice. I got a little big-headed for a day. Again, thanks.

You just reminded me of how much of an effect that MEF has had on me too. I was finally repairing amps on my own and started to develop troubleshooting abilities. Even then I am sure I was reading something already posted that answered certain questions. Now these days when I have a “problem” amp I still start a thread. Only thing is that it is usually some dumb thing that I did like “oh this transistor was installed backwards, no wonder it’s all jacked up”. And who do you think installed this transistor? I plead the 5th.

I need to donate soon but like you I have spent so much lately on cars & medical. But I swear I will soon as this site is very important to me.