Diezel VH4, switching from EL34 to 6L6 == smoke! -solved-

  • Thread starter Thread starter bluesbreaker18
  • Start date Start date
B

bluesbreaker18

New member
My friend has a brand new VH4 stereo, and has been trying to dial in the perfect tone. This tone quest led us to try 6L6's in place of the EL34's.

==========
Short Story
==========
Replacing the EL34's with 6L6's resulted in smoke coming from the small trim-pot mounted near the 1st preamp tube. The PCB reads R3||R5 under the trim-pot that smoked.

==========
A little more info
==========
I'm a hobbyist amp repair tech and have biased many amps, but never a Diezel. Obviously I watched the VH4 biasing video at http://diezel.typo3.inpublica.de/tech-corner.3.0.html before doing this anyway. The only difference I expected from my friend's amp (from the vid) was that his is a 50w amp (stereo), so 25-30mA per side on each OT is approx the target (assuming 480VDC or so).

After looking inside the chassis, I noticed 2 trim-pots. 1 located near the power transformer (PT), and the other located near the 1st pre-amp tube (V1). So I hooked up the DMM to 1 side of 1 of the OTs and measured the current. 36mA. I adjusted (slightly) the trim-pot near V1, the tube's bias wasn't affected. I adjusted (slightly) the trim-pot near the PT, and the tube's bias changed. So I adjusted the trim-pot so that 0 mA were flowing thru the tube. The we shut off the amp. Removed both sets of EL34's. Replaced them with 2 matched sets (NOT a matched quad) of 6L6 tubes. Turned the amp on, let the tubes warm up, and turn standby to the play position. Immediately the trim-pot near V1 smoked. My friend immediately reverted the amp into standby mode.

==========
Verdict?
==========
So, is the 2nd trim-pot, near V1 burned? Is it responsible for the bias of the 2nd OT? Any idea what happened?
 
bluesbreaker18":3h6y4v08 said:
My friend has a brand new VH4 stereo, and has been trying to dial in the perfect tone. This tone quest led us to try 6L6's in place of the EL34's.

==========
Short Story
==========
Replacing the EL34's with 6L6's resulted in smoke coming from the small trim-pot mounted near the 1st preamp tube. The PCB reads R3||R5 under the trim-pot that smoked.

==========
A little more info
==========
I'm a hobbyist amp repair tech and have biased many amps, but never a Diezel. Obviously I watched the VH4 biasing video at http://diezel.typo3.inpublica.de/tech-corner.3.0.html before doing this anyway. The only difference I expected from my friend's amp (from the vid) was that his is a 50w amp (stereo), so 25-30mA per side on each OT is approx the target (assuming 480VDC or so).

After looking inside the chassis, I noticed 2 trim-pots. 1 located near the power transformer (PT), and the other located near the 1st pre-amp tube (V1). So I hooked up the DMM to 1 side of 1 of the OTs and measured the current. 36mA. I adjusted (slightly) the trim-pot near V1, the tube's bias wasn't affected. I adjusted (slightly) the trim-pot near the PT, and the tube's bias changed. So I adjusted the trim-pot so that 0 mA were flowing thru the tube. The we shut off the amp. Removed both sets of EL34's. Replaced them with 2 matched sets (NOT a matched quad) of 6L6 tubes. Turned the amp on, let the tubes warm up, and turn standby to the play position. Immediately the trim-pot near V1 smoked. My friend immediately reverted the amp into standby mode.

==========
Verdict?
==========
So, is the 2nd trim-pot, near V1 burned? Is it responsible for the bias of the 2nd OT? Any idea what happened?
Click to expand...

Was the master switch on the top of the chassis engaged from EL34 to 6L6.....??
V.
 
Seems like on the VH4, the bias trim is near the midi board. The other trim pot if for hum adjustment and you shouldn't need to mess with it.

Steve
 
When referring to video, please look at it again. Pots facing you, bias trimmer is on the left side, hum trimmer right.

Hum trimmer may be burnt, change it for another 100R type. Just leave some of the old trimmers wire and solder the fresh there.

set bias to 30mA for EL34s (yes, with the VH4S you will read a single tube on each side of each output transformer) or 35mA for 6L6.
 
Hey Everyone,
Thanks for your quick responses. This was my fault. I told my friend (amp tech) that there was no switch from EL34 to 6L6. I did not see the switch when I took the chassis out. By the time it was flipped over, it would be almost impossible to spot because it is between a cap and a power tube.
 
Thanks for the responses -- this is good news.

So where can we get a replacement 100R trim-pot for the hum adjustment? This is a smaller value than most bias adjustment pots, which are usually in the 5k - 50k range. Anybody know of a vendor?
 
:clap: Sometimes it's the most obvious things we kinda screw up on...

V.
 
It is NOT a bias pot - you can get them at Mouser or similar.

Alternatively solder 2x 100 Ohm resistors instead of the trimmer.
It is a hum control located at the ground of the heaters.
 
My friend & amp tech (bluesbreaker18) fixed the VH4S earlier this evening by soldering two 100 ohm resistors instead of the trim. Thanks to everyone for guidance.

:rock: :)
 
You must log in or register to reply here.
Back
Top