New build, OD needs help, Solved, clip added

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Bob-I
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New build, OD needs help, Solved, clip added

Post by Bob-I »

Guys, it's been awhile since I built a D-style amp but I decided to give this a go.

I have an amp I built years ago, high plate, no HRM, 2 6V6 output. This amp sounds amazing and I'm trying to duplicate that magic but with reverb. It's squeezed into a Champ chassis in a little head cab. I debated building a combo cab but I still need to hook up the loop and reverb to make it useful.

I have an old Allen chassis, Magnetic Components xformers and all the parts I need to complete this. The clean side sounds great, nice and rich, single tube reverb is good, tone stack effective, but the OD sounds muddled and very one dimensional. Low notes are fuzzy and indistinct, high notes thin, chords sound like mush with no note definition. I've tried several tubes and checked all voltages, which I think are in range.

Plate load resistors are RN65, caps are xicon 630MPP, other resistors are 1W C, Filters are Nichion. except for the main filter with is a cedist can 50/50 in the rectifier hole. These are all the same as I used in my older amp except for the filters, I used 2 cans in the old amp, discrete filters on the new. I tried a few component swaps, smaller OD entrance cap, bright cap on the master, snubbers from 270pF down to 120pF to removed completely. These change the tone but not the muddled tone.

Attaching the layout which has all the voltages listed on it (yes I know the output section isn't complete and the rectifier board and relays are missing). And for reference the old amp's layout as well.

Any help is appreciated.

EDIT: I'm thrilled with this amp now. I recorded this clip so everyone can hear the results.

Recorded on a Tascam ambient recorder. Speaker is a WGS ET65 well broken in, guitar is a Hamer Vector with Duncan Designed PUs, nothing special just one that was hanging on the wall. Give a listen and thx again for pointing me in the right direction to get this one rockin'!!
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Last edited by Bob-I on Thu Jan 04, 2018 3:32 am, edited 1 time in total.
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norburybrook
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Re: New build, OD needs help

Post by norburybrook »

well , the obvious thing would be the reverb circuit as that's the big difference between the two amps. Can you bypass the reverb and see what the OD is like?

I presume you've tried swapping the OD valve?

I don't know how much gain that reverb circuit cuts but on my wonderland build with reverb , the circuit cuts a lot of gain. This is fine as it's a 100w clean amp. I've not built an ODS with verb so can't comment on that side of it, but it something that sprang to mind.



Marcus
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martin manning
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Re: New build, OD needs help

Post by martin manning »

Reverb sounds good in clean mode, so it's not likely that there is a problem there. The reverb is between the preamp out and PI, as if it were in the loop? I wonder if the OD has a parasitic oscillation. Have you tried scoping it Bob?
10thTx
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Re: New build, OD needs help

Post by 10thTx »

I can not enlarge your current layout well enough to see it, but I noticed on your previous layout that the coupling cap after the first gain stage in the OD section is .02 where most of the Dumble schematics how a .01 there. Have you tried a .01? Maybe change the coupling caps in the OD section to Orange Drop PS series?

The other thing that may be worth trying is to use a .01 MusiCap (and maybe a .0047 Musicap in the OD section)? I use MusiCaps sparingly in one of my current builds including the .01 coupling cap after the first gain stage in the OD section.

https://www.ebay.com/itm/Capacitor-Musi ... Sw-0xYTvAz

Reply #2 from this thread: http://el34world.com/Forum/index.php?topic=19534.0
The Musicap was/is the most balanced to my ear, top/middle/bottom end, very nice sounding, maybe the nicest mid range, had the most 3D depth of sound, smoothest distortion and had the best ringing sustain smooth growl/swrill on a full cord left to ring out. I think it has/lets the most harmonics through? It's also, unfortunately, the most expensive.
OR change the .1 coupling caps after the LTPI to .047 if you are using 6V6's.

Just a couple of ideas to consider. with respect, 10thtx
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Bob-I
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Re: New build, OD needs help

Post by Bob-I »

Thx for the replies folks.

Yes Martin, the reverb is after the loop. Marcus, I did bypass the reverb, the OD tone doesn't change although the overall volume drops some.

10th, I'll try some coupling cap changes. I don't think I have all the 6PS caps I need but I've found the MPPs to be very good in these amps, a bit more aggressive which I like, but it doesn't lose the basic character.

I'll see if I can record some clips too.
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norburybrook
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Re: New build, OD needs help

Post by norburybrook »

My thought's were; on clean it doesn't matter so much if you're loosing some level but going into the OD stage that would change matters.
It's strange you lose volume when bypassing the reverb, I would have thought it would be the other way round.

Do you have a trimmer on the OD entrance? I can't see the drawing properly...looks like it might be by the valve rather than on the board. what value are you using?

I presume if you're other amp sounds fine you're using the same settings, so forget that. :D

Marcus
mojotom
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Re: New build, OD needs help

Post by mojotom »

Did you use the same diodes on both amps, same CL2 out cap and PI entrance ?
It seems strange that you're unable to cope the same OD tones only by the verb addition. I would try to remove the reverb from the power supply and signal part to check if that's the case.

Do you have a schem of the first amp ? I can't see much on the layout.
I'm interested in what you did to mate a high plate preamp to a 6V6 output stage, still tweaking mine :shock:

After trying a Two-Rock type of Reverb (Fender verb and return as a mixer) I move to a #60 type of reverb.
In that amp he used fixed resistors giving the same load as a master pushed (470k/470k to gnd) including the mixer of the verb (send to reverb before the master and return after it) plus a master at the end of the mixer part before the PI.
Less loading.
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Bob-I
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Re: New build, OD needs help

Post by Bob-I »

norburybrook wrote: Mon Jan 01, 2018 5:55 pm My thought's were; on clean it doesn't matter so much if you're loosing some level but going into the OD stage that would change matters.
It's strange you lose volume when bypassing the reverb, I would have thought it would be the other way round.
Sorry if I wasn't clear, it is the other way around, adding reverb loses some volume, probably due to the reverb level control loading the PI
Do you have a trimmer on the OD entrance? I can't see the drawing properly...looks like it might be by the valve rather than on the board. what value are you using?
If you click the drawing you'll get full resolution. The OD trimmer is on the board and is 100k. I swapped it yesterday because the first one measured at 67k, the new one measures 102k. I've adjusted it from very low to very high, still sounds wrong. I don't remember where the old amp is set and I don't want to open it up if I don't need to.
I presume if you're other amp sounds fine you're using the same settings, so forget that. :D
Marcus
I'm open for anything and appreciate you taking the time to look at it.
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Bob-I
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Re: New build, OD needs help

Post by Bob-I »

mojotom wrote: Mon Jan 01, 2018 6:43 pm Did you use the same diodes on both amps, same CL2 out cap and PI entrance ?
It seems strange that you're unable to cope the same OD tones only by the verb addition. I would try to remove the reverb from the power supply and signal part to check if that's the case.

Do you have a schem of the first amp ? I can't see much on the layout.
I'm interested in what you did to mate a high plate preamp to a 6V6 output stage, still tweaking mine :shock:

After trying a Two-Rock type of Reverb (Fender verb and return as a mixer) I move to a #60 type of reverb.
In that amp he used fixed resistors giving the same load as a master pushed (470k/470k to gnd) including the mixer of the verb (send to reverb before the master and return after it) plus a master at the end of the mixer part before the PI.
Less loading.
Click on the layout for full resolution. I have removed the reverb fro the signal chain but not the Ps rail, good suggestion.

I did nothing unique to mate the high plate with 6V6s. The first amp was a conglomeration of things I had laying around, a high plate board, champ chassis and Princeton iron. It all fit and sounded great from the day it was born. I was fully expecting to need to make changes.
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Bob-I
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Re: New build, OD needs help

Post by Bob-I »

martin manning wrote: Mon Jan 01, 2018 11:04 am Reverb sounds good in clean mode, so it's not likely that there is a problem there. The reverb is between the preamp out and PI, as if it were in the loop? I wonder if the OD has a parasitic oscillation. Have you tried scoping it Bob?
Well this is disappointing. I used my scope to trace the signal through using a 1K sine wave with no problem. Today I pulled out the old Tecktronix 475 scope to look for PO and there's no horizontal sweep. Looks like it finally crapped out.

I did look for vertical sweep and didn't see anything more than I'd expect to get with basic white noise of a tube amp. Any suggestions on looking for PO without a scope :(
Aaron
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Re: New build, OD needs help

Post by Aaron »

Hi Bob,
As 10thTx mention, try a .01uf, I also noticed your Drive pot is 250kL, maybe try an Audio or even 100kL.
Some have used 1uf on the last gain stage to tighten up the OD.

Aaron
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martin manning
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Re: New build, OD needs help

Post by martin manning »

Well, that's a shame. Your 475 is probably repairable, but you probably weren't looking for that kind of project :^(

Maybe you could run a clean sine into the input and use a listening probe, or plug a cable into the loop return and probe with that? Or, maybe try running the preamp out into another amp to see if the problem is still there?
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Bob-I
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Re: New build, OD needs help

Post by Bob-I »

martin manning wrote: Mon Jan 01, 2018 11:01 pm Well, that's a shame. Your 475 is probably repairable, but you probably weren't looking for that kind of project :^(

Maybe you could run a clean sine into the input and use a listening probe, or plug a cable into the loop return and probe with that? Or, maybe try running the preamp out into another amp to see if the problem is still there?
I used my listening probe. Every stage sounded fine until I hit V2b. I'm not seeing the typical voltage drops you see from PO.

Also, turns out the drive is 100K, typo in the layout. I'm reworking some lead dress on V2 and see if that helps.

Also I had the house alone and cranked it up, very weak compared to what I'd expect from 2 6V6s and the clean side doesn't break up at all. I'm back to the reverb being the problem, will revist that another day.
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erwin_ve
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Re: New build, OD needs help

Post by erwin_ve »

Hi Bob, maybe a bad pot?
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Bob-I
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Re: New build, OD needs help, Solved!!!

Post by Bob-I »

Well, I think I've got it. Mojotom got me going in the right direction. I removed the reverb from the power rail and the OD sounded exactly like my old amp ( a little darker because there's no bright cap on the master), plus the volume was what I'd expect from 2x6V6s.

So I traced the entire reverb circuit and low and behold, I found a mistake. Colorblind people (me) should always measure resistors, so I had my daughter tell me the colors. I had a 47R on the reverb out where there should be a 470K, it was loading down the PI. Replaced the resistor and wow, what a difference. It even improved the clean tone which I was already happy with.

Thx everyone for your help.

As you were...

Next, gotta make a front panel.
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