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FDP Forum / Fender Amps: Vintage (before 1985) / SFDR Harsh Break up


usa, nj

"You guys are great Here's 50 bucks!"
Feb 2nd, 2016 09:24 AM   Edit   Profile  

Since I had a power tube swap and some caps replaced I am missing the sweet overdrive I am used to. The harshness boarders on shrill. SFDR 1968 drip w celestion green back. I know this is subjective but any theories or advice is welcome

Contributing Member

It's not just good..

...it's good enough.
Feb 2nd, 2016 09:44 AM   Edit   Profile  

What tubes were replaced - and what were they replaced with?

Try putting the old ones back in and see if the "sweetness" returns.

Steve Dallman
Contributing Member

Merrill, Wisconsin

Age is just a number...mine is big
Feb 2nd, 2016 10:25 AM   Edit   Profile  

Did you check the bias when you put in new power tubes? If the new tubes are running cold, crossover distortion could be bad.

This amp, unless modified only has a bias balance, not an actual bias adjust. It's easy to add a bias adjust to the bias balance.

I remove the 15k resistor usually on the body of the bias balance, and replace it with a 4.7k resistor in series with a 20k or 25k multi-turn trimpot.

I will also take the braided ground wire off pin 8 of each power tube, and insert a 1ohm, metal film resistor to easily measure the voltage drop across this resistor, which directly translates to the current draw of the tube.

Balance can be set by ear...the lowest hum means the tubes are balanced.

Then set the bias.

My guess is that the previous tubes were running in the right bias range, or even hot, and the new tubes are running cold.

Contributing Member

It's not just good..

...it's good enough.
Feb 2nd, 2016 10:57 AM   Edit   Profile  

"This amp, unless modified only has a bias balance, not an actual bias adjust."

Is that really true of a '68 DR, Steve?
I thought the 68's were the same circuit as the BF models.

I know my '68 has a "true" bias adjust and, as far as I know, it wasn't modified.
Although, it HAD been recapped and a 3-wire power cord was installed. Maybe I should check with the guy I bought it from to see if he did change the bias circuit.

In any case, good call on the hot/cold bias settings.


usa, nj

"You guys are great Here's 50 bucks!"
Feb 4th, 2016 05:00 PM   Edit   Profile  

tubes swapped

2 older lafayette 6v6 power tubes out. 2 tungsol 6v6 gt reissues in.

TAD 12au7a TAD 7025wa tungsol 12ax7 in

rectifer mesa 5ar4 out TAD GZ34 in.

Contributing Member

So. Cal. USA

Feb 4th, 2016 05:40 PM   Edit   Profile  

Even the AB868 DR had adjustable fixed bias same as the AB763 DR. This should be an easy to adjust amp. If you don't have a bias probe installing the 1 ohm precision resistors as Steve suggests is a good idea.

I think putting a 12AU7 in the phase inverter slot could effect tone. Try a 12AT7 in there as called for. Changing the rectifier tube can create some changes in bias as it could effect plate voltage. Sounds like it may be biased cold as mentioned above.

amp mad scientist


Feb 12th, 2016 09:58 AM   Edit   Profile  

Sounds like bias is too cold...agreed.


usa, nj

"You guys are great Here's 50 bucks!"
Feb 21st, 2016 05:54 PM   Edit   Profile  

I swapped the old rectifier back in and it improved a bit. Picked up two nos GE 6V6's and brought it to the local tech for re biasing. Out go the new issue tung-sols.



Measure twice. Cut once
Mar 4th, 2016 10:46 AM   Edit   Profile  

power tubes of even the same brand can be fickle. They usually always take some tweaking.
What caps were replaced??

Contributing Member


We had the hit but Van got the money
Mar 5th, 2016 01:18 PM   Edit   Profile  

That TAD GZ34 was probably sending 10+ VDC more to the power supply vs the 5AR4. Check the B+ and see how far off it is vs schematic. Those amps weren't designed to run at today's 120 - 125vac wall voltage, more like 117vac. Doesn't sound like much but it's multiplied 4x inside the amp. Might try using a buck transformer ahead of the amp, like the Amp preserver or Browner.

Next, I'd look for something better than the Tung-Sol RI 6V6s. I've tried a few sets in PRs & DRs and didn't care for them. You can still find NOS RCA 6V6GTs and 6V6GTAs at reasonable prices. Best 6V6s I've tried. Note that a lot of other brand old production 6V6s were actually RCAs, so look for the octagon logo.

FDP Forum / Fender Amps: Vintage (before 1985) / SFDR Harsh Break up

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