Silverface Twin Balance to Bias Mod

Bandmaster, Bassman, Champ, Concert, Deluxe, Harvard, Princeton, Pro, Super, Tremolux, Twin, Vibrasonic, Vibrolux, Vibroverb and others

Silverface Twin Balance to Bias Mod

Postby Jamie on Thu Dec 17, 2015 5:52 pm

I have a 74 Silverface Twin that was converted over to Blackface to some degree years ago by a tech. The balance pot is as original and I'd like to modify to a bias pot. I found this on Premier, are there any red flags to this process to look out for? Thanks!

http://www.premierguitar.com/articles/20025-ask-amp-man-better-biasing-for-silverface-twin-reverbs
Jamie
Baron
 
Posts: 3223
Joined: Sat Oct 06, 2001 7:01 am
Location: The Man Knows

Re: Silverface Twin Balance to Bias Mod

Postby stratele52 on Thu Dec 17, 2015 9:23 pm

Wich blackface ? Are you sure your tech did not convert balance pot to adjustment pot

Find the layout and follow wiring , Any Blackface with adjustment pot will work .


When you finish your mod , remove Output Power tubes before power amp ON.

Power amp ON
Put a meter on pin 5 on each 6L6 to read if you have at least -36 Vdc .
If not turn your pot until you have it .
If you can read this voltage AND read less or nothing , don't put 6L6's , they 'll red plating ,

Check your circuit , something wrong

Show us some pictures of your pots like Premier Guitars do , whit good pictures we can help you .
stratele52
Knight
 
Posts: 1904
Joined: Sat Aug 21, 2010 9:35 am
Location: Québec province, Canada

Re: Silverface Twin Balance to Bias Mod

Postby stratele52 on Thu Dec 17, 2015 9:51 pm

Do your pot wiring like the first sketch where you see a blue wires

http://el34world.com/charts/bias_conversions.htm
stratele52
Knight
 
Posts: 1904
Joined: Sat Aug 21, 2010 9:35 am
Location: Québec province, Canada

Re: Silverface Twin Balance to Bias Mod

Postby Dean Jr. on Thu Dec 17, 2015 10:00 pm

Jamie, do you have any pictures of the circuit...particularly in the phase inverter area? the reason I ask is that 'blackfacing' entails working with that bias circuit. IF one does not address that issue, there is perhaps little that should be done...if anything at all....to 'blackface' a SF Fender, imho. IF one is going to blackface that amp, then there are a number of changes to be made to the pre-phase inverter area as well as directly to the PI and biasing sections. One might want to look at the power supply area as well....got pics of the filter caps under the doghouse?
Some would leave the circuit as is and simply add a bias voltage adjustment pot in between the bias voltage supply and that balancing pot. That way, one can adjust both the voltage and the balance. IF one found some mismatched tubes, then the balancing act would allow one to use them. I used to totally eliminate the balancing act and all PI related and power supple resistors. The last one I did, I left all of that intact and simply added a bias voltage adjustment trim pot.....worked out great.
Dean Jr.
Page
 
Posts: 314
Joined: Fri Apr 24, 2015 7:59 pm

Re: Silverface Twin Balance to Bias Mod

Postby Jamie on Thu Dec 17, 2015 10:37 pm

Thanks guys. The balance pot is still wired as original, master v twin. Main filter caps were changed last decade, they are fine. The amp starts to distort at about 3 1/2 or 4 which is not right. There were a few mods done and removed. Master volume was removed. The vibrato is a switched pot that takes the vibrato circuit out of the signal path. HT fuse post added. Little fuzzy on the changes done, it was in the early 90s I believe.

Image

http://i1233.photobucket.com/albums/ff386/bfmcgee/sft_board_zpsukogb149.jpg
Jamie
Baron
 
Posts: 3223
Joined: Sat Oct 06, 2001 7:01 am
Location: The Man Knows

Re: Silverface Twin Balance to Bias Mod

Postby stratele52 on Fri Dec 18, 2015 10:19 am

Amp distort early ;

Fix amp before mod bias pot . Did you ear amp was clean before ? When it was out from the tech shop ?
Please report
Few mods done and remove :( ; check carefully the job , if circuit is right .

1-It must be not a bias issue , but check it first .
2-Check all tubes with new tubes ,tubes tester are not reliable for this purpose ,chance is tube tester will show you are tubes are good and they are not .
3-Phase inverter circuit ; if bad resistor , = distort early.
4-Wrong power supply voltage= distort early.
stratele52
Knight
 
Posts: 1904
Joined: Sat Aug 21, 2010 9:35 am
Location: Québec province, Canada

Re: Silverface Twin Balance to Bias Mod

Postby Jamie on Fri Dec 18, 2015 3:01 pm

Thanks for the input guys. I'm mostly a Marshall player. The twin gets very little use, hoping to get to it after the first of the year into spring. I'll probably retain the ht fuse post. The other mods can go. Tubes checked out on my B&K, odd you say that tube testers are unreliable. IIRC the distortion started before the mods were done. It's not bad, it's like it starts to break up at about 3.5 to 4 and it is quite loud. I have 2 coffee can sros in it, weighs a ton.
Jamie
Baron
 
Posts: 3223
Joined: Sat Oct 06, 2001 7:01 am
Location: The Man Knows

Re: Silverface Twin Balance to Bias Mod

Postby jaywalker on Fri Dec 18, 2015 6:26 pm

I'd be inclined to replace both of those white 50uF/70v bias caps while you're in there. (looking at aa270 schemo).
(Positive to ground) and you may end up deleting one after conversion. I like the suggestion to keep the balance.
jaywalker
Baron
 
Posts: 2373
Joined: Sun Jul 03, 2005 1:11 am
Location: California

Re: Silverface Twin Balance to Bias Mod

Postby stratele52 on Fri Dec 18, 2015 6:38 pm

Most of tubes testers test tubes at low voltage and no load .
Tube's amps tubes work at high voltage some with I call heavy load .

With you tube tester it like you are testing if a car is good just listening the engine at idle ,no road test . Can we say the car is good ?
stratele52
Knight
 
Posts: 1904
Joined: Sat Aug 21, 2010 9:35 am
Location: Québec province, Canada

Re: Silverface Twin Balance to Bias Mod

Postby Jamie on Fri Dec 18, 2015 7:10 pm

stratele52 wrote:Most of tubes testers test tubes at low voltage and no load .
Tube's amps tubes work at high voltage some with I call heavy load .


Thanks again guys. I do understand what you are posting. I test for basic emissions, leaks, shorts etc. I don't have access to an AVO or similar so I get by with the B&K or my Precision testers.
Jamie
Baron
 
Posts: 3223
Joined: Sat Oct 06, 2001 7:01 am
Location: The Man Knows

Re: Silverface Twin Balance to Bias Mod

Postby stratele52 on Fri Dec 18, 2015 7:20 pm

Jamie wrote:
stratele52 wrote:Most of tubes testers test tubes at low voltage and no load .
Tube's amps tubes work at high voltage some with I call heavy load .




. I test for basic emissions, leaks, shorts etc. I .


That is what I 'm talking about :laff:
stratele52
Knight
 
Posts: 1904
Joined: Sat Aug 21, 2010 9:35 am
Location: Québec province, Canada

Re: Silverface Twin Balance to Bias Mod

Postby PA100 on Sat Feb 06, 2016 6:30 am

I did the mood on my PA100 ( is the same layout) so I have balance and bias. Easy mod. I just installed a 20 k trim pot and a 8k resistor in series in place of the 15 k resistor. To start with I set the trim pot at 7k so I had the same 15k. Then switch on and adjust away.
PA100
Peasant
 
Posts: 44
Joined: Sun Jul 05, 2015 12:33 pm

Re: Silverface Twin Balance to Bias Mod

Postby stratele52 on Sat Feb 06, 2016 8:57 am

Mod balance pot to bias adjustment pot;


http://www.premierguitar.com/articles/2 ... in-reverbs
stratele52
Knight
 
Posts: 1904
Joined: Sat Aug 21, 2010 9:35 am
Location: Québec province, Canada

Re: Silverface Twin Balance to Bias Mod

Postby pdf64 on Sun Feb 07, 2016 9:29 am

I prefer to retain the bias balance, as per PA100s method; plus it seems a retrograde move to increase the grid referencing resistors to 220k, as suggested for the premier guitar method (it's fine / beneficial to leave them at the lower value, eg 68k, 47k).
The HT fuse is a good thing.
Useful maintenance would be to replace all the white electrolytic caps, especially those in the bias supply.
The cathode resistor on V3 usually drifts up a fair bit, which helps to cool that stage off a little.
I suggest it be replaced with the BF value of 2k2, bypassed or not depending on how much reverb you want, ie bypassed for intense reverb.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1243
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Silverface Twin Balance to Bias Mod

Postby Jamie on Sun Feb 07, 2016 3:25 pm

Thanks pdf and guys for your input. I haven't got to this yet, hoping to tackle it this spring. I'll leave the balance pot as is and add the bias pot, replace the 50uf caps as well.
Jamie
Baron
 
Posts: 3223
Joined: Sat Oct 06, 2001 7:01 am
Location: The Man Knows

Next

Return to Fender

Who is online

Users browsing this forum: No registered users and 3 guests