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Old 21st April 2008, 10:54 PM   #1
christw is offline christw  United States
diyAudio Member
Join Date: Apr 2008
Default Rebiasing my DSL-100

I'm still (sort of) new to tube stuff and I was wondering if someone could tell me what precisely I'd do to switch the output of my Marshall DSL-100 from EL34's to KT66's. I'm doing it on a whim and hoping in combination with some speaker changes, it'll tweak it a wee bit.

Basically, it's too shrill, I'm blaming the Vintage 30's for some of it, but some of my other speakers still are too bright. The damn thing's brighter than a Fender. My Rickenbacker 360 will KILL you with mid/highs right now. Even ordinarily dark guitars still have piercing highs. Right now, I'm sure it's got great bite for a metal setup, but I'm looking to take it down a notch to a more bluesy zone.

I just don't know what exactly to change to rebias it. I know I'm looking to cool it down a bit, darken the tone, and I wanna see if KT66 (going back to the older Marshalls) would top off what I'm looking for.

I have a schematic, but I don't know exactly what to do where. I've done stuff before, replacing shot parts, etc, but when it comes to the modification, I'm not to keen on what to do unless someone tells me. :P (This applies only to amps. )
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Old 24th April 2008, 02:47 AM   #2
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Join Date: Oct 2004
Location: Adelaide South Oz
You need to read that schematic in conjunction with this one:

PR1 sets the bias for V5 and V6 (the "push" side) and the idle current for this side can be calculated by reading the voltage across R6.

PR2 sets the bias for V7 and V8 (the "pull" side) and the idle current for this side can be calculated by reading the voltage across R9.

Here is some explanation (sorry if you understand all this and I'm teaching grandma to suck eggs).

Assume the high voltage is +450V ( measure at pin 3 of any of the EL34s) to 0V - CAREFULLY, with a multimeter set on the 1000V DC range.

The "high voltage will jump across to your finger about 1 inch per 1000 Volts method of measuring is very definitely to be avoided"

Guitar Amps generally biased such that output tubes are running at 70 to 75% of their dissipation ratings.

Rating for EL34 is 25 Watts
25W / 450V = 56mA so would assume that they probably say set bias to give 50mA per tube.

2 tubes on each side so at idle (no input signal) there will be 100mA through each of R6 and R9 and there will therefore be be 100mV across each of these resistors.

To put in a new set of tubes:
Best to use matched quad but because there is a single adjustment for each side you will at a minimum need to use a matched pair for V5 and V6 and similarly for V7 and V8.

With output tubes removed:
With the amp ON measure the DC voltage at pin 5 of either V5 or V6 to 0V and adjust PR1 for the maximum -ve voltage that the adjustment will give. Always a good idea to check that the voltage varies with the adjustment at pin 5 BOTH V5 and V6

Similarly measure volts at pin 5 of V7 and V8 and adjust PR2 for maximum -ve voltage.

This sets the amp up so that when the new tubes are fitted they will have minimum idle current.

Turn Off the amp, fit the new tubes. Connect your multimeter across R6 and the turn on the amp.
If you read more than 100mV turn off immediately you have something wrong.
Adjust PR1 to give 100mV across R6

Turn Off the amp. Wait 5 minutes for high voltage capacitors to discharge. Connect multimeter across R9, turn on the amp.
If you read more than 100mV turn off immediately you have something wrong.
Adjust PR2 to give 100mV across R9

These adjustments will initially have some interaction (because as you turn up the idle current on one side the high voltage will drop a bit and so the idle current on the other side will drop).

Repeat but this time allow 10 to 15 minutes warm up and then do the adjustments.

Hint: You will note that R6 and R9 have one side connected to the tube cathodes and the other side going to 0V. As the final trim you can connect the multimeter from the cathode side of R6 to the cathode side of R9. Let the amp warm up for 15 minutes and then adjust either PR1 or PR2 (doesnt matter which BUT only adjust one of them) to get a reading of exactly 0V. That will mean the 2 sides are exactly matched in idle current.

KT66 have lower gm (mA per volt) than EL34 and so it is possible that you may not have enough adjustment range on PR1 and PR2.
I think it will be OK
If you can't get the currents low enough (that is you need more -ve bias volts at max setting) then R77 can be changed from 10K down to 1K.

If you can't get the currents high enough (that is you need less -ve voltage at min setting) then R68 can be changed from 33k down to say 22K.

Sit down and look at the 2 circuits so you understand how PR1 and PR2 adjust the -ve DC voltage on the grids of the output tubes.

When changing output tubes, always check that with tubes out the adjustment varies the voltage on the grid and then set for maximum
-ve voltage (minimum idle current setting). Then put in the new tubes and adjust the current up to the desired idle point.

A long rave but hopefully that gives you enough info to get the job done.

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Old 24th April 2008, 03:36 AM   #3
diyAudio Member
Join Date: Oct 2004
Location: Adelaide South Oz
Sorry - I made a mistake above
50mA per tube will give about 22.5 Watts dissipation per tube.
Thats a little high for guitar amp use (OK for HiFi).

I would bias at 40mA per tube.
That means 80mV across R6 and R9 NOT 100mV as stated above.

Tubes will last longer that way.

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Old 24th April 2008, 02:23 PM   #4
christw is offline christw  United States
diyAudio Member
Join Date: Apr 2008
Thanks a bunch.

Hopefully I'll be able to pull it off next weekend.
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Old 25th April 2008, 02:02 AM   #5
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Join Date: Oct 2007
In the two 50 watt Marshall's that I have and that we use with my band I have the Saratov KT66's. They sound great and they are an interesting cross between the sound of an EL34 and a 6L6GC. We seem to think that they have the best qualities of both of those tubes which is really great. Keep in mind that we're using old (1974) non master volume Marshall amps and the choice of output tube might make a much larger difference than what it does in a more "modern" amp.

The bias pot might have to be rescaled to get enough range for the KT66 but possibly not you'll just have to experiment. In my notes the one particular amp runs 382 volts B+ and with the KT66 it requires about -43 volts to be properly biased.

Check out this thread where I experimented with converting this same amp to run with 6V6 output tubes.
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