PPIMV ?

[Buschman]

My amp starts to break up @ 9 oclock on the dial. That just can't be output tube distortion. It seems to have more pre-amp gain than any vintage style(read that as no cascaded stages) Marshall, Fender or Vox that I have played. Iain't skeered! I already have a hole where a cut control was. I have to try it. I was hoping that someone had actually messed with the different styles & would have input.

I have been playing around with 6v6s. Seems to knock it down just a bit. I found a set of old Sylvanias that sound good. The jjs almost sound like el34s-lots of presence.

I will give an update on my restrictor plate.

[Firestorm]

Hey UR12--
Thanks for the input. When you tried using the 250k trim pot in your Liverpool, did you wire it as a rheostat or as a voltage divider with the V2 grid connected to the wiper? I know about the RC interaction there -- in a stock Express, I think the corner frequency may be as high as 520Hz. My thought was to leave the basic .002/150K intact, but tap the signal somewhere along the 150K (from the pot wiper). I don't think that would affect frequency as much as a rheostat would -- mostly just signal attenuation. I'll have to play with it. Thanks again.

[UR12]

Hey UR12--
Thanks for the input. When you tried using the 250k trim pot in your Liverpool, did you wire it as a rheostat or as a voltage divider with the V2 grid connected to the wiper? I know about the RC interaction there -- in a stock Express, I think the corner frequency may be as high as 520Hz. My thought was to leave the basic .002/150K intact, but tap the signal somewhere along the 150K (from the pot wiper). I don't think that would affect frequency as much as a rheostat would -- mostly just signal attenuation. I'll have to play with it. Thanks again.

To be honest, it was so long ago, I would have to pull the chassis and look. Just give it a go and let us know how it turns out >

[Buschman]

I installed the Rich mod PPIMV. I think it's going to do what I need. No it does not sound good a bedroom volumes. I don't play in my bedroom-especially on Valentines day. It does sound great at near full volume which just made my Express more versatile. I used a 1 meg pot & shielded cable to the el34s.
I don't think lightening will strike me but is is supposed to rain all weekend.

[Doug H]

My experience with attenuators and master volumes is they are good for a few db and that's about it. IMO a 30-100w amp is never going to sound good at bedroom volume just through attenuation (or a master volume which is a form of attenuation).

[jem]

3. Change the 220K grid return resistors on the output tubes to 150K or even 100K. This cleans up Fender Deluxe Reverbs like magic. How would it impact a TW?
Any guinea pigs out there? Okay, I'll shut up now.

Guinea Pig here
That suggestion to change the grid return resistors is interesting to me.

I have a '68 Deluxe Reverb and an Express that I'm still tweaking to get it "just right". (the DR is far from original , the PT blew up about 20 years ago, and the speaker was replaced. I'm afraid it has the dreaded noisy-fiberboard-rot thing going on.)

If the grid return resistors are changed, that will affect the bias correct? I'll need to rebias? Just wondering if this would be a good place to adjust or switch for different tone shapings.
Will the bias circuit need to be changed as well?

Thanks

[Firestorm]

If the grid return resistors are changed, that will affect the bias correct? I'll need to rebias? Just wondering if this would be a good place to adjust or switch for different tone shapings.
Will the bias circuit need to be changed as well?

Thanks

Jem--
Changing the grid return resistors does NOT affect the bias. There is almost no current flowing in the bias circuit so the resistors have virtually no impact there. What they do affect is a)signal to the output tube grids because the bias supply is an AC ground and b) the tube's ability to get rid of stray electrons that get "stuck" on the grid. Some tubes will self destruct if the grid circuit resistance is too high.
It's an easy experiment since you don't have to change anything else. I use 150Ks in BF Fender Deluxes and in some Super Reverbs (if I am trying to clean them up a bit.)

[jem]

If the grid return resistors are changed, that will affect the bias correct? I'll need to rebias? Just wondering if this would be a good place to adjust or switch for different tone shapings.
Will the bias circuit need to be changed as well?

Thanks

Jem--
Changing the grid return resistors does NOT affect the bias. There is almost no current flowing in the bias circuit so the resistors have virtually no impact there. What they do affect is a)signal to the output tube grids because the bias supply is an AC ground and b) the tube's ability to get rid of stray electrons that get "stuck" on the grid. Some tubes will self destruct if the grid circuit resistance is too high.
It's an easy experiment since you don't have to change anything else. I use 150Ks in BF Fender Deluxes and in some Super Reverbs (if I am trying to clean them up a bit.)

Thanks Firestorm--
So those resistors could be replaced with a 75-100K resistor and 150K pot in series for a dirty/clean type adjustment, or switch another 220K-330K resistor in parallel for the dirty/clean control without causing any harm?
Hmm.

[Firestorm]

The effect of changing the bias resistors to a lower value is pretty subtle, but in a Trainwreck, small changes can make a difference. I would just experiment with the impact of different values: 150K, maybe 100K. I don't think it would make enough difference to be a dirty/clean effect. Ken did have a master volume design (Type 2 from the Trainwreck Pages) where you replace the grid return resistors with a dual 100k linear pot. The bias voltage is fed through the pot regardless of where it's set (so the tubes are safe) but as you turn it down, more of the signal gets shunted to ground through the bias supply. But the consensus seems to be that MVs don't sound all that good in a TW.

If you want a dramatic change to the sound, fooling with the 150K resistor on the grid of V2 is a more likely candidate. Any kind of voltage divider will affect the amount of signal going into V2. I'm going to to do that experiment with a pot eventually, but I can almost already predict that it will work best if the values are just tweaked, not changed wholesale.

I think that one of the keys to the Trainwreck sound is the interaction between V2 and the size of the signal coming from V1. That 100K Ra and 10K Rk combination biases V2 so that it can only pass a signal of about 30V peak to peak before it clips. That's why turning up the volume on your guitar doesn't so much increase the output as change the character of the sound -- V2 has a maximum output of sorts. Below that threshold the signal is clean, above it, it's dirtier (or something).

I think there's also a chance that setting the amp's volume pot high could permit a little clipping in V1b -- but that tube clips the opposite side of the waveform from V2.

There's some very sexy stuff going on in this circuit so it pays to tread lightly and make smallish changes so you can gauge their impact.

[Buschman]

You know man, as bad as I wanted that ppimv to be great, I just feel that the amp lost something. It's coming back out . I now think it is a bit buzzy.

Hey Dana, What did the 100k on the v2 grid do for the tone? Is it a bit less aggressive? Did you leave the pot in the amp?

[RHGraham]

Firestorm


BTW....Welcome to the forum Randal!

Thanks Dana.

I just wanted to say a powerscale set-up isn't a mastervolume, if it's applied to the output tubes only, and the compensation controll is added ( which is a post-pi MV, but used more as a balancing controll), there is no reason why the wrecks couldn't be turned back a little bit, 1, 2 or even 3 db, with no fundamental change in tone or interaction. It simply regulates output power, not the interaction between pretubes and powertubes, the rectifier, or anything else. Just the actuall output power.

Being able to take a couple of db out of the top would allow you to have the fundamental trainwreck tone, but also with the controll that allows you to balance with other band members and rooms, and keep soundmen and club owners happy too.

Once you're a star you can be as loud as you want, in the 2000 seat plus clubs. That ain't most people's reality though, is it.

I agree no amp sounds good at bedroom levels ( whatever that is), but I also think a few db off the top, being able to do that, is essential for an amp you're going to gig with. Maybe for your hearing as well.

all you toughguys that do the "all men play on 10" routine, just don't have tinnitus yet.
You're really gonna love it...

[Doug H]

I agree no amp sounds good at bedroom levels ( whatever that is), but I also think a few db off the top, being able to do that, is essential for an amp you're going to gig with. Maybe for your hearing as well.

all you toughguys that do the "all men play on 10" routine, just don't have tinnitus yet.
You're really gonna love it...

I agree, which is why I think it's better to just design it for lower power to begin with. Attenuators/PPIMV's/etc are good for a few db. I attenuated a liverpool proto I was playing with and for 3-6db it sounded fine. -9db was the limit though. Below that it was flat & lifeless. Eventually the speaker is not moving enough air, Fletcher-Munson takes over, etc, etc...

[UR12]

You know man, as bad as I wanted that ppimv to be great, I just feel that the amp lost something. It's coming back out . I now think it is a bit buzzy.

Hey Dana, What did the 100k on the v2 grid do for the tone? Is it a bit less aggressive? Did you leave the pot in the amp?

No the smaller the resistance the less agressive the amp is. The Liverpool originally had a 68k in that position. I think a 150k, like The Express was just a little too much. I do think it changes the tone just a bit but with the trim pot you can adjust it for the tone you like and if you can't get there with the pot you can always try changing the .002 cap to something else.

Yes, I left the pot in there.

[RHGraham]

I agree, which is why I think it's better to just design it for lower power to begin with. Attenuators/PPIMV's/etc are good for a few db. I attenuated a liverpool proto I was playing with and for 3-6db it sounded fine. -9db was the limit though. Below that it was flat & lifeless. Eventually the speaker is not moving enough air, Fletcher-Munson takes over, etc, etc...

exactly, I'm at the same place with you on that completely, I don't think in terms of being able to turn it all the way down, but a little adjustment overall I think is a good thing, but without altering the flat-out tone... I've listened to a lot of the clips here and the TW's obviously have thier voice when run that way, just like a whole bunch of other amps do.
And building it with lower output is just fine as well.

I like the idea of scaling or some other method, so you could have an amp that would cover the different venues, rooms, etc, and go along with changing conditions as well. There were lots of gigs where my little deluxe was a little too loud for the first set, but not quite enough for the third, it was a real aggravation.

I don't think scaling is the ONLY way to do it, but it's a good option, and from what I've tried lately, I don't believe it would affect the fundamental tone of a TW circuit enough that it couldn't be compensated for by the controlls on the amp.

[John_P_WI]

There was a forum member several months ago, DerStever, who modded his with power scaling. I believe it is working well, maybe he will jump in and elaborate.