Modded 2061x build thread

[Colossal]

have to dig up a semi-scaled VVR schematic from somewhere, i think i have the old "power scaling the 18W amp" from the 18watt.com prooject with me. would an IRF740B handle this amp? that's what i have with me at the moment.

thanks again!

EDIT: Colossal, I have no idea what you mean by the 1M resistors and caps... did i miss something?

Hi Flood,

Yes, there are some additional steps you must take when scaling the whole amp using a VVR arrangement. You need to change the order of the grid stopper and grid leak resistors on the input of V1 as well as add another cap/grid leak to the grid of the second stage. Traditionally, you would have the input jack first, followed by 1M grid leak resistor wired input to ground, followed by the grid stopper of your choice (typically 10k-68k), then into the grid.

If you are scaling voltages throughout the whole amp using a VVR, you need to add an additional cap directly to the grid of the input to prevent DC from "backing up" onto your guitar volume pot when the VVR is scaled way down. The order then becomes: input jack to grid leak resistor, to a film cap (roughly the same value as your output section so, 0.022µF is fine), to grid leak resistor wired to ground, and then into the grid of V1. Ideally, the cap and 1M to ground can be soldered directly to the tube socket. Similarly, you add another cap and 1M to ground on the grid of the second grid (after the coupling cap if AC coupling). I've enclosed a PDF showing this schematically. These drawings were taking directly from the installation manual Dana Hall supplies with his VVR board.

In case you are wondering if you can do without the caps, I would advise against it as I have tried it and yes, it does put DC on your guitar knob and it does get annoyingly scratchy. If you are scaling the power section only, then the above does not apply however you then must separate the B+ feeds to the preamp and power section using diodes.

I hope this helps.

[flood]

hey colossal,

thanks for the VERY useful information... i didn't know about that problem at all.

i'm looking at the option of scaling the power amp and the PI only, leaving the preamp tubes at the higher voltage. would i still need the additional measures? couldn't hurt to integrate them, i suppose.

re: the amp - i plugged it in and let loose and it worked pretty well on both channels. i do have a weird feeling that the power output is a bit lower than expected though, but then i am getting only about 300V on the EL84 plates, as opposed to the 350 recommended in the schematic.

there is one rather nasty problem though: i included a cascade switch. this is essentially a 3PDT wired to:
1. route the output from the lead channels 1M volume resistor into the bass channel via a 220k resistor while
2. disconnecting the bass input jacks and
3. shorting the lead channel's 220k cathode follower mixer resistor, input side, to ground.

the distortion sounds fizzy, gated and almost synth-like. it's very hard too describe. almost like a tame version of the infamous metasonix f***ing f***er. there seems too be some clean signal bleeding through as well. wwill try and record it for you guys.

it could be lead dress or the fact that there is no shunt resistor to ground to act as a voltage divider between the cascaded channels. not sure; will have to troubleshoot that. but not tonight. no more tonight.

[Colossal]

hey colossal,

thanks for the VERY useful information... i didn't know about that problem at all.

i'm looking at the option of scaling the power amp and the PI only, leaving the preamp tubes at the higher voltage. would i still need the additional measures? couldn't hurt to integrate them, i suppose.

Hi Flood,

You're welcome but don't thank me, thank Dana (UR12) as he did all of the hard work! If you are just planning to scale the power section here is an excerpt from Dana's manual which is of immediate use for your application:

"<b>Regulating the Power Tubes Only</b>

This method is preferred by some, just like regulating the whole amp is preferred by others. This method will split the power supply into 2 different parts. One part is regulated, which supplies voltage to the power tube plates and screens, and the other part is left normal to run the preamp tubes. This method will also require you to make an additional mod to the amp. You will need to add a Master Volume so that the signal coming from the preamp tubes running at normal voltages won’t overdrive the power tubes running at reduce voltages. The general consensus is to add a Post Phase Inverter Master volume or PPIMV. This works well in a Push Pull amp but there is no Phase Inverter in a Single Ended amp and a normal Master Volume control will work fine between the last preamp stage and the power tube(s). You will also need to install 2 1N4007 diodes to isolate the two different sections of the power supply so they don’t interact with each other"
-- D. Hall, VVR2 Installation Manual

I've attached some drawings from the manual which should help you visualize what you would need for this installation.

FWIW, the amp that I installed a VVR in (a 6V6 plexi), I scaled the whole amp. I also added a PPIMV just to see how the tone would be affected. I found the VVR to be very useful as a means to control the overall sound pressure while preserving the essence of the amp's tone up to a point. This is what it was intended for...controlling stage volume, not getting perfect tone down to bedroom levels. It does exactly what it is supposed to. Even with the VVR, I still played the amp loud to get to the sweet spot. The VVR took the edge off. I did not however care for the master volume or the effect it had on the amp. A master volume however is necessary if you are scaling the power amp only as you will be slamming the PI if you turn down the voltage to the power section without limiting the signal going to the PI from the preamp.

re: the amp - i plugged it in and let loose and it worked pretty well on both channels. i do have a weird feeling that the power output is a bit lower than expected though, but then i am getting only about 300V on the EL84 plates, as opposed to the 350 recommended in the schematic.

Yes, lower voltage would change the tone. You might want to check your rectified voltage to make sure the PT is putting out what you expect.

there is one rather nasty problem though: i included a cascade switch. this is essentially a 3PDT wired to:
1. route the output from the lead channels 1M volume resistor into the bass channel via a 220k resistor while
2. disconnecting the bass input jacks and
3. shorting the lead channel's 220k cathode follower mixer resistor, input side, to ground.

The added complexity of this arrangement may have some tradeoff in terms of overall quality. I tried a similar switching arrangement which allowed the amp to run in either parallel or cascade but was only using one input jack. I ran into issues with noise although I was very careful with lead dress and shielding. I think the additional lengths of wire running from the tube socket routing signal were not a good idea and contributed to the problem. I also ran into oscillation problems with mixing the channels. Although I was able to work out the issues, I eventually scrapped the idea as it was more problematic than it was worth. For what it is worth, I just built a JCM800 style amp that is set up correctly as a cascaded amp and it is way, way quieter than the other amp using the cascade/parallel switch, has none of the oscillation problems, and sounds just way more massive. It is like a very mean, musical chainsaw

the distortion sounds fizzy, gated and almost synth-like. it's very hard to describe. almost like a tame version of the infamous metasonix f***ing f***er. there seems too be some clean signal bleeding through as well. will try and record it for you guys. It could be lead dress or the fact that there is no shunt resistor to ground to act as a voltage divider between the cascaded channels. not sure; will have to troubleshoot that. but not tonight. no more tonight

It would help to see a picture of how you have the switch set up. It sounds like something strange is going on. The distortion tone will depend too on how the tubes are biased and cathode bypassed but the gated quality sounds a bit odd. With the setup I used, the Bright channel volume knob became a Gain control when the amp was in cascade mode so there was some signal shunted to ground.

Good luck sorting it out. In the meantime you might want to take the switch out of the equation, making sure the amp sounds good in one mode or the other. If you can get that going, then that will help you troubleshoot where things could be going wrong.

[flood]

Colossal, thanks a lot again. the next few days should have a fair amount of troubleshooting scheduled...

here are the schematics: the stock soultone schematic by Michael Corrieri (i hope i am allowed to repost this, if not please let em know and i'll take it down) as well as my somewhat primitive switching scheme. i am not sure if it is 100% accurate with resüpect to grid stoppers though, will have to look at the amp again. i doubt i would connect the grid of the bass channel directly to the switch with nothing in between.

again, thanks for the assistance and support. i doubt i would have started building amps if i hadn't come across this forum.

[flood]

bump.

[Colossal]

Colossal, thanks a lot again. the next few days should have a fair amount of troubleshooting scheduled...

Flood, sorry for the lateness of my reply. Have you made any progress sorting out your issues?

here are the schematics: the stock soultone schematic by Michael Corrieri (i hope i am allowed to repost this, if not please let em know and i'll take it down) as well as my somewhat primitive switching scheme. i am not sure if it is 100% accurate with resüpect to grid stoppers though, will have to look at the amp again. i doubt i would connect the grid of the bass channel directly to the switch with nothing in between.

The switching scheme seems odd to me in that I noticed the absence of grid leak resistors, but then perhaps it's implied that the 1M-to-ground is connected to the jacks (not shown) for each set of inputs. Also, I noticed in your switching scheme that you cathode biased the bass channel and lead channels separately (more like a plexi) but on the Soultone scheme both channels are sharing a cathode resistor and bypass cap (fully bypassed).

When you are in cascade mode, I'm wondering if the 220k/220k mixing resistors are not enough to keep the channels from interacting. On a plexi they are typically 470k/470k however some use 220k/220k but are a bit more interactive. Also, a typical plexi is blending channels, not running them in cascade. The 470k value may cause some treble attenuation but, in my experience, plexi amps are plenty bright to begin with. I'm also guessing that when you are in cascade mode, the 1M volume pot on the Lead channel forms a divider with that 220k grid stopper you show going into the bass channel which would give some needed attenuation.

Also, on the Soultone scheme (that you attached, but not the one on their website) there is an 0.1uF cap in the PI that is polarized. That might just be a typo.

Have you been able to do some troubleshooting? Personally, I would get the amp working perfectly without the cascade/parallel switch at first. When you get that happening, then bring in the switch and see where you get the problem. I too had a plexi that I could switch from parallel and cascade and found that it was troublesome and just set it up for parallel and ditched the cascade. I just built a JCM800 style amp which is cascade by design and it worked perfectly the first time and is very quiet.

You've probably already done this, but use shielded cable on anything going to and from V1 and that switch and double check any ground schemes. Once you get the amp working correctly (without the switch) you might try jumpering the bass and lead channels to get the plexi vibe.

Good luck.

[flood]

Hey Colossal,

thanks a ton for all your help so far. i actually did sort out the issues!

i made four changes in total:

1. lead channel cathode bypass resistor - i had 1k5//22uF - changed to 1k5//680nF (ERO)
2. bass channel plate resistor - 100k 2W bypassed with 1nF ceramic.
3. added a 220k to ground in the switching path. it's 220k/220k now, earlier that 220k you see in my switching scheme was just floating around unreferenced...
4. (probably the root of all the issues) replaced the wire from the cascade switch to the bass channel input with shielded wire. it was a fairly long run, and i'm quite certain that it was causing issues, especially since this amp has horrifying lead dress. the chassis got mucked up on drilling (the front panel drillings got "inverted" so that there wasn't sufficient space to put in a turret board between front panel and tubes, so they had to go behind - it's part turret (flea) board and part "true" point to point now) but i figured i might as well run with it instead of throwing it away...
4a. minor mod - the lower cap on the bass channel tone control was originally to 68nF (why?!? because i have a bunch of 68 nF styroflex caps and just had to use one somewhere!) - i brought it down to 22nF, which is still double the specified 10nF.

how does it sound now? sweet! noise, fizz, oscilllations are gone. i'm not getting the searing leads i was hoping for in cascade mode, but then i don't think this amp was ever designed to get into 2203 territory, seeing that it lacks a meaty CF stage.

it's not exactly a 2061x anymore, but i love it... and so did this bogner + soldano owner who came over yesterday. i'm quite kicked about that, since this is my 4th amp. i don't even have a cab right now - my 2 ax84 1x12s are waiting to be covered and finished before i can get them home.

i think i will post pictures of the insides soon. resistors are all metal film/metal oxide, caps vary but are mostly NOS german wimas and EROs. electrolytics are indian keltrons, the only reliable current production caps i can get in bombay... i wonder how samsung caps are.

samples once i get my mic stand back!

[flood]

The switching scheme seems odd to me in that I noticed the absence of grid leak resistors, but then perhaps it's implied that the 1M-to-ground is connected to the jacks (not shown) for each set of inputs.

that's right - the 1M resistors are on the jacks. the 68k grid stoppers are also directly soldered to the jacks.

Also, I noticed in your switching scheme that you cathode biased the bass channel and lead channels separately (more like a plexi) but on the Soultone scheme both channels are sharing a cathode resistor and bypass cap (fully bypassed).

yeah, i imagined this wwould be necessary after devising the cascade mod - i didn'T know if the shared RC bypass would lead to a problem, so i split that up right from the start...

When you are in cascade mode, I'm wondering if the 220k/220k mixing resistors are not enough to keep the channels from interacting. On a plexi they are typically 470k/470k however some use 220k/220k but are a bit more interactive. Also, a typical plexi is blending channels, not running them in cascade. The 470k value may cause some treble attenuation but, in my experience, plexi amps are plenty bright to begin with.

i think i might try upping those resistors, since in cascade mode, there is a lot less gain than expected even with the lead channel maxxed. it sounds like... i had put a slight boost before the bass channel, no huge difference to speak of.

when cascade mode is activated, the 220k resistor that would normally connect the lead channel output to the PI is connected to ground. so we have an additional voltage divider here. i'll make one change before upping these resistors, namely...

i could bypass the bass channel output resistor, but it's quite bright already. it should be ok as it is.

I'm also guessing that when you are in cascade mode, the 1M volume pot on the Lead channel forms a divider with that 220k grid stopper you show going into the bass channel which would give some needed attenuation.

i added a 220k resistor to ground, as sstated earlier, from the 220k grid stopper. i'm guessing this attenuates things even further - possibly more than necessary? will just have to try it and see.

You've probably already done this, but use shielded cable on anything going to and from V1 and that switch and double check any ground schemes. Once you get the amp working correctly (without the switch) you might try jumpering the bass and lead channels to get the plexi vibe.

you nailed it - all connections except one! that one was pretty critical, i'd say...

oh, and the jumpered sound is sweet... real sweet!


this one is almost done.... just a bit of fine tuning left to get what i want out of it.

[Colossal]

Excellent!! It's great to hear that you got it figured out and the amp is working well. Yes, I noticed that unreferenced 220k. Changing that 330µF cathode bypass cap to 680nF should tighten the low end up considerably. You might lose some of that classic plexi "rumble" but I noticed for instance running my 2204 based amp into a pair of greenbacks in a closed cab, the low end is really tight...more of a modern feel to it. It has a low wattage 680nF film cap on the first stage cathode and the clarity is a lot better than my plexi with an electrolytic in that position.

Using shielded cable solved a lot of my troubles when I had that mod in my plexi amp. I had a particularly vexing oscillation that stopped immediately when I changed out one wire to shielded. The mixing resistors were particularly microphonic. I've since removed the mod however as I just preferred having the amp in parallel mode more.

Cheers.

[flood]

hey there,

you think it would be ok to up that 220k to 470k or even 1M? i'd like a bit more gain (i'll probably try it even if you say no just to see what happens ) and distortion on the whole.

the bass channel still retains the "330"uF capacitor (which in my build is a mix of three different caps... 220uF//100uF electrolytic, 68n styroflex). would you recommend swapping this out for a 47uF/63V non polarized styroflex? or is the 330uF ok?

microphonic resistor never heard of that one before. was it metal/carbon?

thanks!

[Colossal]

you think it would be ok to up that 220k to 470k or even 1M? i'd like a bit more gain (i'll probably try it even if you say no just to see what happens ) and distortion on the whole.

Sure, give it a shot. Your amp is not a stock 2061 anyway so just experiment till you find the tone you are looking for

the bass channel still retains the "330"uF capacitor (which in my build is a mix of three different caps... 220uF//100uF electrolytic, 68n styroflex). would you recommend swapping this out for a 47uF/63V non polarized styroflex? or is the 330uF ok?

You are probably just fine the way you have it. I don't think you would notice much difference in the low end once you get past 22uF as that is fully bypassed anyway but using film caps in place of electrolytics will tighten the low end audibly. However, the degree to which that will be noticeable given how much capacitance you have there may be subtle. But going from 330uF of electrolytic like in a plexi to 0.68uF film only like a 2203 *is* quite audible. Part of the old school plexi vibe is in the low end...the looser rumble, so if that appeals to you and you like the sound of the bass channel as you have it...it's all good then. Especially if you plan to run the amp in parallel for the plexi sound. As you have noted, it is sweet! I think the different voices complement each other well, one is a bit too low, the other a bit too high, but together...they are just right!

microphonic resistor never heard of that one before. was it metal/carbon?

I should have clarified that statement a bit more. The *region* around the mixing resistors was microphonic. I also had a VVR in the amp which required the addition of another cap and a 1M grid leak at the junction of both mixing resistors and the V2 grid. Tapping the cap produced a loud *thunk*. I adding a piece of shielded cable there that cured an oscillation.

[flood]

so, here are some clips for all you great people on these boards - this amp is SWEET! you can listen to them at http://www.soundclick.com/bands/default ... tent=music

[flood]

i took apart the amp because it was suddenly dead and practically rebuilt it using tag board, this time with a LOT cleaner lead dress than before, star grounding pre- and poweramp, and also took the opportunity to add a VVR (using IRF840 according to the 18W lite schematic).

i have two problems now:

1. oddly enough, it hums more now! this despite the fact that all heater wires are better twisted and are now flat on the chassis, well isolated from signal wires. i'm just wondering - i used one of the bolts on the mains transformer as the star ground point, but grounded a shield on the primary as well. could this be causing a ground loop? i'd think that grounding the shield wire would be quite necessary. the other thing is that the shield wire was grounded directly to the lug of the mains socket and not the star ground, something i'm not going to think too much of, since i'm using THICK wire from the lug to the star ground point, and the wire length is 2 inches or less.

i could always disconnect the shield and find out.

2. the volume pot on the lead channel crackles a LOT more than usual, even with no guitar plugged in. i will eventually upgrade my pots from the horrible 16mm alpha pcb mount types (which are, unfortunately, the best pots available locally) but i suspect that the pot isn't the only reason. i did use a 100n DC blocking cap after each pot, as some people have recommended when using a VVR.

3. the IRF840 is rated at 500V 8A. would i need to heatsink it anyway?

the VVR seems to be working fine, although it doesn't get as quiet as i had hoped (dimed at 2 am without waking the folks ). i think i'm going to replace the 100k resistor to ground from the 1M VVR pot with a 33k resistor and see if that helps.

thanks again.

[flood]

ok, so i lifted the shield wire from the ground point and the hum increased even more. i'm nonplussed. i'm going to lift the star ground lugs from the trafo bolt and see if that helps, could be a chassis problem or a loose joint at the ground point. will possibly solder the lugs together or something.

it seems the IRF840 does heat up considerably, so i guess i'm going to have to heatsink it to the chassis.