Diagram of the DR103 voltage doubler bias mod?

DR103, DR103OL, DR504, DR504OL, DR103S (Gilmour), CP103 (Townshend), Lead 30, Lead 50, Lead 100, DR201, DR405, SA112, SA212, SA412, SE4121, SE4122, SE4123

Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Mon Apr 13, 2015 7:42 pm

Does somebody have a diagram of the DR103 voltage doubler bias mod? I can't read schematics :| Thanks
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Sun Apr 26, 2015 5:53 pm

Does anyone have pictures they could share?
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby Vince95 on Wed Apr 29, 2015 3:01 pm

Hello DLP4

I modified two DR103 with bias pots (dual bias mod). I found the schematic on google from MHuss but I couldn't set the bias correctly for JJs or others.
So I modified it. I'll post pictures soon when I get time to access to my FTP. I also modified the schematic accordingly and I have the layout for you.
If you want to have picture before I post it on FTP send me your email address by private message and I send them tonight.
two '76 Hiwatt DR103 (with bias)
1978 Hiwatt SA112 (with bias)
1986 Marshall 2203
1996 Fender Twin Amp
TAD JTM45 clone
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby ian frankish on Wed Apr 29, 2015 3:22 pm

I built a DR103 a few years ago and included the Mark Huss bias mod which worked perfectly. I only have one picture which i doubt you would be able to see exactly what is going on but for what it is worth the bias mod can be seen on the extended pillar boards in the picture. The obvious things are the two blue electrolytics and the variable potentiometer which is a lighter blue.

Image
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby Vince95 on Wed Apr 29, 2015 3:39 pm

Maybe it works with only one bias pot, it didn't with dual bias pots.

Here are the layouts I used.
Original hiwat (remove the red components)
http://sgtmckenzie.free.fr/hiwatt/DR103vintage.pdf

modified bias : add the red components
http://sgtmckenzie.free.fr/hiwatt/DR103withdualbias.pdf

I will send pictures later
two '76 Hiwatt DR103 (with bias)
1978 Hiwatt SA112 (with bias)
1986 Marshall 2203
1996 Fender Twin Amp
TAD JTM45 clone
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Wed Apr 29, 2015 8:05 pm

ian frankish- thank you. I found your pictures and studied them but like you said it's kinda hard to see.

I studied the Huss single and double bias and not being familiar with schematics, it's a bit confusing. Like it shows what I believe is the transformer negative bias voltage and ground going to both ends of the caps and resisters. I'm trying to get it but not there yet. There was a 100w volt doubler diagram posted by Erickson Audio that uses different caps and resistors and a 50k pot that I'm trying to make sense of.

Vince95- thanks, I PMed my email. I'm going to look thru your links. One bias is good. If the tubes are matched that should be good enough.
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby Vince95 on Wed Apr 29, 2015 10:37 pm

I understand you want to use single bias since it's easier to modify your amp but matched tubes are not that much matched.

See pictures in the link below. I am not showing them in the forum because they are too big (2000 px).
Note that the caps have been changed as well to new ones. Don't keep 40 years old caps...

http://sgtmckenzie.free.fr/hiwatt/bias_1.jpg
http://sgtmckenzie.free.fr/hiwatt/bias_2.jpg
http://sgtmckenzie.free.fr/hiwatt/bias_3.jpg
http://sgtmckenzie.free.fr/hiwatt/bias_4.jpg
http://sgtmckenzie.free.fr/hiwatt/bias_5.jpg
two '76 Hiwatt DR103 (with bias)
1978 Hiwatt SA112 (with bias)
1986 Marshall 2203
1996 Fender Twin Amp
TAD JTM45 clone
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Thu Apr 30, 2015 8:35 am

Thank you! I appreciate your help
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Wed May 06, 2015 7:35 am

When I bias the tubes, what's a safe start setting on the pots so I don't damage anything?
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby Mr. Foxen on Fri May 08, 2015 12:33 am

Max negative voltage and bring it back up from there.
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Fri May 08, 2015 8:01 pm

ok thanks
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby Vince95 on Thu May 14, 2015 1:09 pm

So how did it turn? Did you get your amp correctly biased?
Don't hesitate to mesure the bias current on the four power tubes, you may find variations.
two '76 Hiwatt DR103 (with bias)
1978 Hiwatt SA112 (with bias)
1986 Marshall 2203
1996 Fender Twin Amp
TAD JTM45 clone
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Fri May 15, 2015 5:54 pm

I ordered parts but I haven't received all of them yet. Thanks
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby dlp4 on Mon May 25, 2015 9:28 pm

I got it wired up but I did something wrong. I'm reading no bias voltage at all regardless of how the pot is adjusted. Plate voltage is 544VDC. The plate voltage use to be 473VDC. I'm guessing the plated voltage is jacked up because of no bias voltage.

I'm using the RV4LAYSA253A 25k ohm pot. The one with the locking nut. I was careful. I confirmed the band markings on the resistors and checked the pot resistance and double-checked the wiring so now I need to go hunting. If anyone has thoughts please let me know. thanks
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Re: Diagram of the DR103 voltage doubler bias mod?

Postby Vince95 on Tue May 26, 2015 7:41 am

Attach detailled pictures and schematic of the wiring you decided to use.
Check the polarity of the caps.

Did you put tubes in the sockets ?

Check again if you removed all the components. Check value of resistors with an ohm meter
(You know the caps keep the voltage to a lethal level for several minutes don't you? unplug the power plug and wait a few seconds for the caps to drain, leave the standby switch on ON to drain faster - check voltage before)
two '76 Hiwatt DR103 (with bias)
1978 Hiwatt SA112 (with bias)
1986 Marshall 2203
1996 Fender Twin Amp
TAD JTM45 clone
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