THD picovalve

[gnappi]

I just bought one at a silly low price in a local pawn shop and it's pretty unusable as it is beyond 3-4 on the preamp control.

I've diddled with low gain pre-amp tubes (12AU7 and 12 AT7) as well as a power tube assortment from 6l6, 6V6, and 6CA7's and nothing cleaned it up, so it's pretty clear that the stage gain of the tubes has nothing to do with the ugly (for my purposes) sound.

I cleaned up the original schematic which was a fuzzy jpg, to a .gif and from what I've read about mods online removal of C1, C7, can be removed
and removing Dev 1 (feedback?)

I have these side thoughts possibly adding low value cathode bypass caps on V1-1 V1-2 and V2-1 and raising the cathode resistor V2-1 from 820 to 1.5k?

I also find it odd that the power tube is run AC while the pre tubes are DC.

Thoughts and suggestions?

[nickb]

C1 rolls of the the signal above about 3.5Khz, I estimate. Removing it won't have much effect as that is quite a high frequency. You can try, it won't do any harm.

I couldn't find C7 on the schematic so I can't comment on that.

The use of "DEV" instead of "R" or "C" isn't especially helpful. Resistor DEV12 is the slope resistor for the tone stack is nothing do to with feedback. It is absolutely required for the tone stack to work properly but you can play with the value a little. Search for Duncan's Tone Stack Calculator - a great tool for that sort of thing.

The preamp tubes have DC heaters to lower the hum. You don't get much benefit using DC on the later stages and they have a greater current requirement and so would put greater demands on the power supply.

WHen you say it sounds nasty, have you tried turning VR5 all the way up and backing off the VR1 and vice versa.

[gnappi]

Thx for the reply.

C7 is at VR1 and C2, I have not mentioned R12 and it's understood that it's required, but DEV1 seems to be a feedback loop. I agree the component nomenclature is weird and not mine. Perhaps they are not OEM supplied? Dunno I've never seen another print other than the one I found and cleaned up.

Yeah, there's zero headroom with the master at most any position especially all the way up, and the gain control past 3-4 becomes useless . It sounds worse than a cheap fender SS amp with an overdrive switch.

I plan on playing with the cathode bypass scheme and raising the 820 to 1.5k on V2-1 also.

[Jazz P Bass]

I see a Triode switch on the output screens.

Probably best to keep that off.

[Chuck H]

Voicing the amp (by adding/removing caps or changing other component values, etc.) can happen later. I think the first thing to do would be to change the preamp topology so it's not a dirt box anymore. As to headroom... It's tiny so "headroom" becomes a very relative concept. Adding cathode bypass caps raises the gain even if you change the preamp cathode resistors. So if you want to reduce gain DON'T add bypass caps. Anyway... Changing the topology doesn't need to be complicated. Look at the added switch in red. It removes one of the cascaded gain stages. It may pop a tad when used, so don't switch it while you're recording The amp is probably a PCB but where there's a will there's a way.

[nickb]

Thx for the reply.

C7 is at VR1 and C2, I have not mentioned R12 and it's understood that it's required, but DEV1 seems to be a feedback loop. I agree the component nomenclature is weird and not mine. Perhaps they are not OEM supplied? Dunno I've never seen another print other than the one I found and cleaned up.

Yeah, there's zero headroom with the master at most any position especially all the way up, and the gain control past 3-4 becomes useless . It sounds worse than a cheap fender SS amp with an overdrive switch.

I plan on playing with the cathode bypass scheme and raising the 820 to 1.5k on V2-1 also.

I have no idea why I read DEV12 where you'd written DEV1. It happens. The capacitor DEV1 rolls off high frequencies but it is (1) quite aggressive: e.g. with VR1 mid way it will start rolling off around 400Hz, and (2) There is an interaction between VR1 and the rolloff frequency - viz. as VR1 gets away from the mid position the rolloff frequency will increase. No that there is anything wrong with it but I would choose to do it differently.

Thank for pointing out C7. That's commonly called a bright cap and it purpose is to try to even out the frequency response changes as VR1 is moved. You can play with all these things, but only after you figured the gain issues.

The earlier question about VR1/VR5 settings and the affect on 'nasty' sound was to figure out if it's the preamp of the power amp that is causing you grief. If you have VR5 all the way up and gradually increase VR1 the power amp should break up first. On the other hand, with VR5 well down you can raise VR1 and cause the preamp to breakup. My guess it's the preamp but it's better to do the test so we're not looking in the wrong area. I couldn't really be sure from the description in your response which condition you were in. Hopefully now that you understand where I'm going you'll be able to try again and it will make more sense. Personally I generally dislike the sound that an overdriven single-ended amp such as this makes, but that's just me.

[gnappi]

Ah, I see your point If say the VR1 is at 2 and the VR5 is max it's STILL muddy and distorted but at a low volume.

[nickb]

Ah, I see your point If say the VR1 is at 2 and the VR5 is max it's STILL muddy and distorted but at a low volume.

Well I guess I'd need to do the math but it looks like this is not capable of driving the power amp hard enough to clip so your problem is in the preamp. Try the switch Chuck suggested - he has loads of experience of tweaking to optimise a given amp (and presumably therefore a LOT more patience that I have). If you like the low gain setting you could rewire (in theory but might be tricky with a PCB) to move the now unused stage after the master volume and so reduce the voltage swing on V2/2. It suspect that stage is responsible for much of the nastiness.

[Chuck H]

There are a lot high pass and low pass circuits in that design. Tone can likely be improved a lot. We need to get the gain structure in line first, I think. The tone stack definitely suffers a lot of loss but I think there should still be enough gain to drive the power tube to full output, if not clipping.

It's odd that you would have less "headroom" with the master full up. How are you interpreting headroom? Headroom, for our purposes, would be the amps max clean volume. That will invariably be with the master volume set at max. Regardless of any other knob settings. If, for example, with the mater set at 6 the amp starts breaking up at a preamp volume of 3, but with the mater set full up the amp starts breaking up at a preamp volume of 2 that only means you are clipping the power tube first rather than the preamp. The maximum volume at which the power tube clips has not changed in either case. The preamp volume knob number setting is arbitrary. That is, if the amp breaks up with a preamp volume of 2 but after changes the preamp volume can be increased to 6, that doesn't change in any way how much signal can be put into the power tube before it clips. It only changes the amount of gain. Knob settings and gain are not headroom, per se. I think I know what you're after though. Just keep in mind that the amp will NEVER have a louder clean tone than it does now. Even if that clean tone is only achievable with the preamp volume at .5 and the mater volume full up now, modifications that make it so the preamp volume can be set to 9 for the loudest clean tone will not make the amp louder. It will only change the number setting of the preamp volume knob where the amount of gain necessary to distort the power tube occurs. I really hate explaining this because most people that don't get it in the first place just become confused and despondent. But trust me. Your loudest clean tone, and therefor greatest headroom, is only available with the master volume set full.

[nickb]

I really do doubt that the output can be driven hard enough with the tone controls midway.

You would need about 18Vpp on the grid of a 6V6 for full output. So, with the tone controls midway you would need around 180Vpp on the cathode of V2/2. This is way past the range as which a DC coupled cathode follower is happy. Here Merlin shows that at 20Vpp out things are starting to go wrong, giving lots of even harmonics. A little is a good thing, but lots is just nasty. I would have thought that simply putting a 12AT7 in for V2 should help somewhat as the output swing will be increased.

A scope would be really handy right about now.

PS: I really should have said Vpk not Vpp. It now very obvious that this was designed that only the preamp breaks up and the power stage never does. You could also try connecting the tone stack to V2/1 plate and disconnecting the cathode follower completely - as least it will break up differently, you might prefer it.

[gnappi]

Thx Chuck,

" How are you interpreting headroom? Headroom, for our purposes, would be the amps max clean volume"

Yes, for sure that's it. It doesn't matter whether it's on the 2w or 5w setting, it gets muddy really fast with the master full up and the gain past three or four.

I haven't opened it yet, but I do have a scope so more to come. I worked in an R&D lab for 21 some odd years so will have no problem with mods at the PCB level.

[donluca]

Anyway... Changing the topology doesn't need to be complicated. Look at the added switch in red. It removes one of the cascaded gain stages. It may pop a tad when used, so don't switch it while you're recording The amp is probably a PCB but where there's a will there's a way.

Thanks for this Chuck!
And sorry for bringing up from the dead such an old thread but I'm on the verge of buying the small PicoValve used and wanted to "Championized" it by making as close to the original Fender 5F1 circuit, but with some radical modding.

First, the addition of Chuck's switch (I'll be using the Standby switch for it after disconnecting it since I don't really see much use for it – and besides I've read that it makes a very loud pop every time you engage/disengage it).

Second, the removal of the entire second tube, so that it now becomes the same 12AX7 + 6V6GT topology of the original 5F1.

I'd like to keep the rest as it is because the Master Volume and the triode/pentode switch may come very handy when I play late at night.

This is the modded scheme I've come up with and I have a couple of questions, but first, I'll post the original schematics along with the modded one and the Fender 5F1 schematic so that you can easily compare between them:

Chuck's modified Picovalve:



My "Championized" Picovalve:



Fender 5F1:



Questions:

1 - I've removed C18 and R19 along with the DEV7 resistor because there isn't a second tube anymore, was this the correct call?
2 - Now that there's only one 12AX7 preamp tube, should I make R10 a 22KΩ resistor like in the original 5F1 schematic?
3 - Is R16 part of the tone stack? I'm not entirely sure and I'd rather be safe than sorry before removing it.

Thanks!

[Chuck H]

There are some flaws in your approach here. The Picovalve only has three gain stages. A Champ has two. The switch I added reduces the gain stages to two all by itself. In the Picovalve the triode feeding the tone stack is operated as a cathode follower and has no gain. It's there to reduce signal impedance and loading by the tone stack. Which is necessary in this case because the tone stack here loads the signal a lot more than the Champ tone stack.

Keeping the switch while removing a tube gives you the option of reducing the gain stages from two to only one. I don't think that's very useful since one gain stage followed by that tone stack, especially without the cathode follower, will not be enough drive for the power tube grid. So if you're set on removing a tube from the circuit you can also remove the switch.

By removing DEV7, C18 and R19 you are removing the power supply filter for the power tube screen grid. You would also be reducing power supply resistance feeding the first preamp tube (which you noted). You should leave DEV7, C18 and R19 in place whether you remove a tube or not.

Not even sure why you would remove the standby switch. If you don't use it then just leave it in play mode. It's existence in the circuit has no tonal affect at all.

R16 is not part of the tone stack. It is the cathode resistor for cathode follower circuit you proposed removing. It would only act as an unnecessary load without the original design that included the cathode follower.

Really nothing about your circuit is more Champ like than simply using the switch to remove a gain stage. The Champ is such a different circuit that I don't see it reasonable to try to make that amp like a Champ considering that you're limited to the topography dictated by the PCB.

[donluca]

Thank you so much for your detailed reply!

As you may have guessed, my experience with tube based amplifiers is pretty lacking, I'm more of a solid state guy, but wanted to give a tube amp a go once in my life to actually feel the difference firsthand and take the opportunity to learn something new.

There are a couple things I'd like to delve a bit deeper though to further my understanding of how this amp works.

The switch I added reduces the gain stages to two all by itself.

That's because the 12AX7 is a dual triode tube, which is basically a "two tubes in one" kind of thing and your switch bypasses half of the 12AX7, right? It's basically like a dual opamp.

Would I achieve the same result just by using a lower gain tube like a 12AY7? (the 12AX7 has a gain of 100, 12AY7 has 45)
IIRC the Picovalve is self-biasing so I don't need to change the bias.

In the Picovalve the triode feeding the tone stack is operated as a cathode follower and has no gain. It's there to reduce signal impedance and loading by the tone stack.

Ahhh, I see! That makes perfect sense. With that in mind, does that mean that whatever tube I put in there (be it a 12AX7 or a 12AU7) won't alter the amount of gain?

By removing DEV7, C18 and R19 you are removing the power supply filter for the power tube screen grid.

I swear, I'm looking now at the schematic it has become obvious, that was pretty dumb of me.

Not even sure why you would remove the standby switch. If you don't use it then just leave it in play mode. It's existence in the circuit has no tonal affect at all.

Because I don't want to drill additional holes for your "half power triode" switch, I'd rather repurpose that one since I'd leave it always on.

R16 is not part of the tone stack. It is the cathode resistor for cathode follower circuit you proposed removing.

That's what I thought when I first saw it, don't know why I second-guessed myself on that point. In fact, the first "revised" schematic I made, it was without that resistor.

Really nothing about your circuit is more Champ like than simply using the switch to remove a gain stage. The Champ is such a different circuit that I don't see it reasonable to try to make that amp like a Champ considering that you're limited to the topography dictated by the PCB.

I see, I initially thought that the cathode follower would be an additional gain stage and would saturate too much the power tube, hence I would always get a distorted sound and I was looking forward a solution where I could get clean tones and slightly saturated bluesy tones at bedroom volume levels.

Thanks again for chiming in, really appreciated! (and if you have some useful resource I could dig into to understand better how tubes and tube amps work, that would be immensely apprciated!)

[Chuck H]

That's because the 12AX7 is a dual triode tube, which is basically a "two tubes in one" kind of thing and your switch bypasses half of the 12AX7, right?

Right.

Would I achieve the same result just by using a lower gain tube like a 12AY7? (the 12AX7 has a gain of 100, 12AY7 has 45)

Not exactly, but always a good thing to try for lower gain since it doesn't involve circuit mods.

the Picovalve is self-biasing so I don't need to change the bias.

Yes. All stages in that amp are self biased.

With that in mind, does that mean that whatever tube I put in there (be it a 12AX7 or a 12AU7) won't alter the amount of gain?

You'll still get a gain reduction because only 1/2 of that tube (two triodes in there) is a cathode follower. The other half is still an amplifier.

Because I don't want to drill additional holes for the switch for your "half power triode" switch, I'd rather repurpose that one since I'd leave it always on.

The switch I added is a gain reduction switch. I think that should be specified since that amp DOES have a triode switch stock. As in triode operation of the power tube. As to repurposing the standby switch, that seems like a good consideration at face value, but it runs long leads into a noisy area of the amplifier to get the preamp to that switch. That could cause unwanted noise or instability. I think you'd be better off using a push/pull pot and fly leads from the lug terminals to the board. A little clunky, but actually better than repurposing the standby switch.

I put something together for you. I'll put it in a follow up post in just a minute.