Overdrive Special build

[erwin_ve]

Just finished a ODS, based on a #102 but with some tweaks.

This is my 7th build but the first one that does the feedback thing right out of the box on living room levels, not as much as Tony's but still audible in clean mode. Really pleased with this one!
Still got to adjust the PI balance.

70-ies Peavey Deuce iron is used for both PT and OT in this build with bridge rectifier for HT.
Hammond Choke(194B) and Hammond relay transformer(186b12).
Mid cap is 0.05uF and for pots on the OD side I used some NOS eighties Allen Bradley type J 100k lin.
Master bright cap is on a CTS push pull pot.
Grid resistor for v1a is a NOS Beyschlag 24k CF resistor.
Also a tantalum presence cap.
All coupling caps are tested on a scope to make sure the outer foil is on the right side.
#124 is used to keep wiring example. Except for the Plate-cathode wiring were I used some common sense in what I think contributes to more feedback.

To do: waiting for a custom made faceplate.

[M Fowler]

Nice job on your build. Is the 102 a lot brighter circuit?

Mark

[erwin_ve]

Thnx Mark!
The master Bright cap is switchable as a push pull.. When using no dlator: no brightcap, no bright amp.
In fact I used the wrong snubbers on V2: 25pF instead of 250pF. But still smooth

[larsvictor]

Hello Erwin,

congratulations to this nice amp. Any chance for some audio clips, especially that desired flipping into feedback?

What do you think is responsible for that feedback thing? I know, this has been discussed extensively in Tony´s "feedback city amp" thread, but as far as I remember there was no answer to that question.
My scratch build #102 doesn´t have it at livingroom levels. Could you successfully mod your other amps to achieve this effect?

Thanks
Lars

[erwin_ve]

Hello Erwin,

congratulations to this nice amp. Any chance for some audio clips, especially that desired flipping into feedback?

What do you think is responsible for that feedback thing? I know, this has been discussed extensively in Tony´s "feedback city amp" thread, but as far as I remember there was no answer to that question.
My scratch build #102 doesn´t have it at livingroom levels. Could you successfully mod your other amps to achieve this effect?

Thanks
Lars

Hi Lars, thnx! I will try to make some clips, busy days ahead this week.
I did several things with the wiring differntly.
Master pot wiring unshielded in parralel to présence cap wiring.
Tonestack wiring exactly done as in #124.
Plate v1a parralel to cathode v1b= positive feedback. Plate v1b is not running parralel to cathode v1b wire because this create negative feedback and is diminishing the audio feedback which I desire so much.
Pi wiring exactly as in #124.
After finishing this amp, I'll give it a try on my other Ods's.

One thing to mention is that' I did some extensive testing with positive feedback by tieing cathodes together. It gave some promising feedback results but never ended up with stability when turning the pots up.

[larsvictor]

Hi Erwin,

thanks for your fast response. Very interesting for me. Just to recap: You would assume that this feedback thing is caused by positive feedback due to special lead dress around the first valve. I´m curious about the result if you tweak another amp the same way. Will look at my amp, especially the wiring around V1. BTW V1b is the first gain stage in your amp?

Lars

[erwin_ve]

Amp is almost done, still waiting on the custom faceplate. Meanwhile the amp is at its new owner: a promising 19yr old guitarist.
A few things had a impact on the feedback:
When increasing the first dropping string resistor there was less feedback.
When changing the cathode bypass caps from Nichicon to Elna silmic(same value) there is less feedback.
When changing the v1 and v2(tube reverse) there is less feedback.

[larsvictor]

Wow, Erwin. That one looks great.
Superb tweed cab job.

Thanks for the additional information concerning the feedback thing.

Lars

[norburybrook]

very nice indeed


interesting comments about the feedback.

Is it 100% repeatable with a different build?


M

[erwin_ve]

Wow, Erwin. That one looks great.
Superb tweed cab job.

Thanks for the additional information concerning the feedback thing.

Lars

Thanks Lars!

very nice indeed


interesting comments about the feedback.

Is it 100% repeatable with a different build?


M

Hi Marcus,
Im gonna rebuilt my personal#102 as soon as possible with things I learned from this build. Yet I only can sum up things that do not contribute to the feedback thing:lol:

[norburybrook]

Erwin it will be interesting to see if you can reproduce it


keep us all posted.


Marcus

[lovetone]

Hi Erwin would there be any chance of a better quality image of the area around V1 please? I have tried to see on your image but there is not much detail there

Thanks

[erwin_ve]

Hi Erwin would there be any chance of a better quality image of the area around V1 please? I have tried to see on your image but there is not much detail there

Thanks

As soon as the amp is back for it's faceplate I'll shoot some pics.

[norburybrook]

Irwin,

I'm building a wonderland soon using Peavey iron from a donor amp.

I'm going to have to do a full wave bridge rectifier too as the transformer is 195-0-195.

what were your transformer specs? and can you show me a detail of your rectifier board, I'm having trouble working out a layout for the board for the bias and main B+ rectified voltage supply. I've only ever used the bi phase , standard methods with regular twin 320-0-50-320 type power transformers.

cheers

Marcus

[erwin_ve]

Irwin,

I'm building a wonderland soon using Peavey iron from a donor amp.

I'm going to have to do a full wave bridge rectifier too as the transformer is 195-0-195.

what were your transformer specs? and can you show me a detail of your rectifier board, I'm having trouble working out a layout for the board for the bias and main B+ rectified voltage supply. I've only ever used the bi phase , standard methods with regular twin 320-0-50-320 type power transformers.

cheers

Marcus

Hi Marcus,
For this amp I used Peavey Deuce iron.
The specs for the pt are approx: for B+: 350vac, full wave bridge is around 480-500v.
Bias tap: 60-0
Filament 6,3 vac

The rectifier board is the same as #102 in the files section, that amp also used full wave bridge.
Make sure you dont you use the center tap for the full wave bridge!