PI Plate Voltages

[Charlie Wilson]

Hello, I have been working on a Low Plate Classic for quite a while now and struggling a bit with high end shrillness especially with a Strat. Recently(yesterday) I received two GE long plate 12AX7s that are matched balanced within a mA. I installed one of them and hit a power chord with my Strat expecting to hear the same kind of bright shrillness and was pleasantly surprised that the harshness was gone and there was some beef to the sound even with the bridge pickup. I measured the PI plate voltages and they were 300v and 307V. The plate voltages of the previous tubes I was using(Mullard reissue, Telefunken) were more in the 285v and 295v range. I tried the second GE tube witch draws slightly more current and had voltages in the 295v and 300v range and it was brighter, still not shrill but definitely brighter. This got me thinking that maybe, at least in my amp, I need to be more critical of the PI plate voltages. Tony's layout suggests that the PI voltages in 124 were around 280v and 290v. My mains AC swings between 117vac and 121vac and that seems to have the biggest impact on the PI plate voltages. I was finding that when my AC was low my amp was shrill and bright and when my AC was high the amp sounded a bit better which was the opposite I would have expected with lower plate voltages on V1 and V2. I would like to hear other peoples thoughts on this issue.
CW

[martin manning]

Robben Ford said recently (in a Premier Guitar interview, I believe) that his Dumble is finicky about line voltage. You could easily test that effect with a Variac. Theoretically you should retune the PI trimmer if you change PI tubes. A balanced tube may not be as well balanced at the actual operating point, and different sellers (with their different testers) may test for balance at different operating points.

[aflynt]

Now that you mention it, my low plate classic was dark and dull sounding with its old JJECC83S PI tube. I switched to a JJ ECC803S and it was much brighter. I tried the old tube in V1 and the plate voltages went up like 12 volts compared to the V1 voltages when I tried the new ECC803S on V1. I put a third tube back in V1 that got the plates around 200. I assumed that the brightness came exclusively from the PI trimmer setting since it does get duller when it's set wrong, but now I'm wondering if the tube itself had more to do with it.

-Aaron

[Charlie Wilson]

Aaron, it's funny we seem to have the opposite issues with our amps in terms of brightness. Maybe I need to try one of those Eminence Delta speakers. I just know that with my amp, guitar, and speaker, I prefer the sound with the higher PI plate voltages. I notice 183 has higher 300+v PI plate voltages. I just seems like most of the comments about preamp tube voltages are very specific about V1 and V2 but kind of general(less than 300v) for V3. Yeah Martin I think it is time I invest in a Variac. I have only been repairing amps for 8 years without one. I purchased the GE tubes from Brent Jesse and he test them at 300v for current draw.
CW

[aflynt]

Do you have a master bright cap and are you using a D'Lator? I'm not using the master bright now that the PI is tweaked and I pretty much always patch in the D'Lator to soften and mellow out the amp. Been using my G12-65 loaded cab for recording. Still darkish but not as much so as the Delta Pro.

-Aaron

[Charlie Wilson]

Aaron, no Dumbleator yet and no master volume bright cap. Thus far the amp never needed one. I may be somewhat beating my head against the wall trying to get the amp to sound perfect without the Dumbleator, so I think shortly I am going to leave the amp alone for a bit and build a couple loops. I am using a EVM 12S speaker so there isn't much treble roll off from that. Oh yeah, I went back to 330pf treble cap from the .002. The .002 is really fun in OD mode at low volumes but I started missing the nice Fenderish chime I was getting with the 330pf. Getting back to the PI, these GE tubes and the higher voltages have gotten me closer to what I want.
CW

[jam-mill]

Aaron, it's funny we seem to have the opposite issues with our amps in terms of brightness. Maybe I need to try one of those Eminence Delta speakers.
[...]
CW

Charlie,

I just put an Eminence Delta Pro 12A into a D-Style 1x12 open back cabinet and while it has some serious 'thump' it is by no means a cure for high-end harshness. My (only) d-clone has high-end 'hash' with OD engaged and I think the only cure is going to be the HF taper (awaiting a parts order to implement).

BTW, the speaker break-in process, using a 60Hz signal was run for several hours to also mitigate any exaggerated high end.

-jack

[ToneMerc]

Hello, I have been working on a Low Plate Classic for quite a while now and struggling a bit with high end shrillness especially with a Strat. Recently(yesterday) I received two GE long plate 12AX7s that are matched balanced within a mA. I installed one of them and hit a power chord with my Strat expecting to hear the same kind of bright shrillness and was pleasantly surprised that the harshness was gone and there was some beef to the sound even with the bridge pickup. I measured the PI plate voltages and they were 300v and 307V. The plate voltages of the previous tubes I was using(Mullard reissue, Telefunken) were more in the 285v and 295v range. I tried the second GE tube witch draws slightly more current and had voltages in the 295v and 300v range and it was brighter, still not shrill but definitely brighter. This got me thinking that maybe, at least in my amp, I need to be more critical of the PI plate voltages. Tony's layout suggests that the PI voltages in 124 were around 280v and 290v. My mains AC swings between 117vac and 121vac and that seems to have the biggest impact on the PI plate voltages. I was finding that when my AC was low my amp was shrill and bright and when my AC was high the amp sounded a bit better which was the opposite I would have expected with lower plate voltages on V1 and V2. I would like to hear other peoples thoughts on this issue.
CW

One of the best things you can do for your own builds, for your own taste is to stop looking at and being tethered to what's written on a layout.

As far as voltages, its not uncommon for me to spend hours just rolling preamp tubes and documenting voltages. Next try to figure out if the tangible differences are really tube brand or simply voltage changes.

There's a difference between an assembler and a builder. Assembling to the point of a working amp is the easy part....LOL.

You can go down a rabbit hole chasing some of this stuff on the bench, but out and about live, these things can become nearly a moot point.

TM

[bluesfendermanblues]

One of the best things you can do for your own builds, for your own taste is to stop looking at and being tethered to a layout.

TM

Yes! we tend to get hung up on a particular amp, which is generously documented by Gil, Scott, Charlie W. and other fine people at this forum, but we sometimes forget, that those amps were build by HAD as part of a system for
-a particular player
- with a certain pick and pick attack
- style and genre
- guitar
and not least speaker cabinet

and that small variations in actual component values and maybe building technique (soldering gun and heat) make - on the surface- identical amps sound very different.

HAD probably spend a lot of time (4 years) tweaking his amps. Therefore, we should probably expect the same time consuming trial and error, as part of the process and not paint too much by numbers.

[jam-mill]

One of the best things you can do for your own builds, for your own taste is to stop looking at and being tethered to a layout.

TM

[...]

and that small variations in actual component values and maybe building technique (soldering gun and heat) make - on the surface- identical amps sound very different.

The above really kind of says it all; so many variations due to the various component tolerances. I think the reverse engineered amps can only be used as a guideline.

This may be D-blasphemy, but this amp reminds me of the early incarnation of the Linux operating system. At its core a fundamentally great system, but required many tweaks to make it usable...

[bluesfendermanblues]

BTW my low plate classic tells me it likes more bottom end in the OD, so I upped the 5n to 10n and the 10n to 22n and the result is much more growl and a nice blues tone with my strat.

I actually think It does the robben thing quite well only with a strat with SC's instead of meaty HB's.

If I should relate the above mentioned mod to dumble terms, you could say its moved towards a 3rd gen design.

It's been said a lot of times on this forum - build one of the documented designs and then again let your ears decide your tweaks - after a lot of play-in time.

[martin manning]

CW, I checked out Brent Jesse's matching test (he has made a video) and it looks like he has a good process. You remarked that of these two GE tubes one is noticeably brighter than the other. Do you have an LCR meter to measure the interelectrode capacitances, especially Cg-k? That is the only thing I can think of that would cause that effect. I measured a few tubes I have and found ~2x variation among a small sample. You have two there from the same manufacturer and construction that show a different HF response, so it would be interesting to see if that might be the reason.

[stelligan]

It's been said a lot of times on this forum - build one of the documented designs and then again let your ears decide your tweaks - after a lot of play-in time.

Sage advice. We all end up wanting to sound like ourselves - like we want, in a way. Getting an amp there takes longer with some builds than others. Eventually you end up with a pile of amps that sound the same - if you're not careful.......

To the PI point - my best result is around 6v difference then listening for best sustained tones on the unwound strings near the nut. D cowboy chord area. Seems that the PI adjustment mostly impacts sustain elements rather than timbre for me....

[Charlie Wilson]

Thanks Guys, yea Mike I guess I am being a bit of a newbie imitator. Although I am starting to loosen up and free form it a little. The funny thing about the layouts are that we don't even know if those are the tubes Dumble installed originally and if they are, they could be 20+ years old. I like what Gil said that he doesn't bother with tube voltages unless the amp doesn't sound good. I am not so sure about every Dumble amp being tweeked for every purchaser. I personally know people that gave money to Dumble, maybe gave him a general idea of what they wanted, waited four years or so and received their amp. I think the personalized tweeking is reserved for the high profile guys(at least for more recent amps). Martin I do not own a LCR meter, I thought maybe different current draw would affect the frequency response of the tube, I think it is time for me to learn more about basic tube theory and tube circuit design. In tweeking my Low Plate Classic to what I like it is starting to look a lot like the Carlos Rios amp. I am really leaning towards a larger than 330pf treble cap and I think I will do it as a .001/500pf mid boost. That way I could still have the chirpy 330pf and a larger .001. I am also thinking having a polyester cap before the ceramic may smooth it a little. I do like the Deep switch but I think the mid boost is a more gig useable option.

[jcsifu]

In tweeking my Low Plate Classic to what I like it is starting to look a lot like the Carlos Rios amp. I am really leaning towards a larger than 330pf treble cap and I think I will do it as a .001/500pf mid boost. That way I could still have the chirpy 330pf and a larger .001.

Charlie,

You might be interested to try this 3 way option Martin came up with. I like having this option and it has proven to be effective for live use when switching guitars or to fit in the mix better.

jc