No bias trimmer on 1999 Diezel VH4?

[Dan Precision]

I am new to this forum, and I have been having some troubles with my VH4. After the amp had been on for a few hours in my studio, a low hum started to come out of the speakers. I made sure that all of my connections were secure. It seemed like the tubes were much warmer than usual, and the transformer on that side was so hot that I couldn't touch it. The transformer on the speaker output side was warm, but not nearly as hot as the other. After shutting the amp down for about 15 minutes and turning it back on, I noticed the hum was still there.

I've had this amp for for at least 4 years, and based on the serial number starting with 99, I'm assuming it's from 1999. Since this amp only gets played around 4 days a month (although it's usually on for 8-10 hours), and the tone still sounds incredible to me, I never changed the tubes. After this problem, I thought that maybe it's time. When I opened it up, removed the amp from the chassis, and turned it upside down, I couldn't find a bias trimmer or the hum trimmer that I have read about. Does this year not have a bias trimmer? It does have the switch up top for 6L6 or EL34.

It currently has Electronic 6550A power tubes and JJs for the entire preamp section. Any help or direction will be much appreciated. Thanks.

...now that I have the amp out of the chassis, I tried it out again. I am hearing a high pitched frequency emanating from the transformer closest to the power tubes. Is that an indication of trouble starting? Once the guitar is turned up, I the hum is barely audible, but if I turn the volume down on the guitar, the hum becomes very apparent.

 

[i.ak]

Beautiful! ...but can't see any trimmers.

A bit OT, but kind of related as it's a tubey thing as well: I have had significant issues with similar 6550/KT88 tube-retainers as you have. They seem to cut to the glass in some occasions.

 

[Jens Kruse]

My personal opinion and advice to you:

First of all after over 4 years I would strongly recommend doing a complete tube change. Get the Diezel hand selected Ruby preamp set and a new set of 6550s (if those are what you prefer).

On the 2nd photo you posted (from top) on the right upper corner of the circuit board, to the left of the screw, you see 2 100 Ohm resistors that are in place of a hum trimmer. I recommend replacing the left resistor for a 47 Ohm/2Watt one.
It is better to have resistors instead of a hum trimmer in VH4 amps as sometimes the trimmer gets fried.

If you are not sure which resistor I am referring to you can email me the 2nd photo and I will mark it for you.

Kind regards,

Jens

 

[Jens Kruse]

Beautiful! ...but can't see any trimmers.

A bit OT, but kind of related as it's a tubey thing as well: I have had significant issues with similar 6550/KT88 tube-retainers as you have. They seem to cut to the glass in some occasions.

To prevent the retainers from cutting into the glass you can buy a rubber ring (see image below).

Jens

 

[duesentrieb]

You can easily replace the resistor underneath the fuse and left to the diode in the bias circuitry with a pot (mounted to the top or back of the amp) and enable proper biasing. I had to do the same when I've owned a 1998 VH4.

 

[Dan Precision]

My personal opinion and advice to you:

First of all after over 4 years I would strongly recommend doing a complete tube change. Get the Diezel hand selected Ruby preamp set and a new set of 6550s (if those are what you prefer).

On the 2nd photo you posted (from top) on the right upper corner of the circuit board, to the left of the screw, you see 2 100 Ohm resistors that are in place of a hum trimmer. I recommend replacing the left resistor for a 47 Ohm/2Watt one.
It is better to have resistors instead of a hum trimmer in VH4 amps as sometimes the trimmer gets fried.

If you are not sure which resistor I am referring to you can email me the 2nd photo and I will mark it for you.

Kind regards,

Jens

Thanks Jens. I appreciate the info. I think I see the resistor you are talking about. Is it the one with the extra smaller resistor attached? If you look closely at the photo, you will see it.

I should have mentioned that the JJ preamp tubes are only about a year old. If I did want to get the full compliment of tubes, can I order them through you?

I've seen some tube websites where you can specify your amps voltages so they can send you tubes that perfectly match your voltage. Would I have to do that since I don't have a bias adjustment?

I was going to email you directly, but I figure other people could benefit from this information as well.

Dan Precision

 

[Jens Kruse]

Dan,

yes, it is THAT resistor with the smaller extra resistor soldered in parallel. Replace those 2 with ONE 47 Ohm/2 Watt resistor.

You can order tested and burned in tube sets from me. Please e-mail me when you are ready.

Instead of ordering the tubes to match the voltage and current of your bias resistors, I would second Olaf's suggestion of replacing the resistor underneath the fuse (your 1st photo, center) with a bias trimmer. I have those in stock and can send to you. After all, you are all about "Precision", correct? ;-)

Kind regards,

Jens

 

[Peter Diezel]

My personal opinion and advice to you:

First of all after over 4 years I would strongly recommend doing a complete tube change. Get the Diezel hand selected Ruby preamp set and a new set of 6550s (if those are what you prefer).

On the 2nd photo you posted (from top) on the right upper corner of the circuit board, to the left of the screw, you see 2 100 Ohm resistors that are in place of a hum trimmer. I recommend replacing the left resistor for a 47 Ohm/2Watt one.
It is better to have resistors instead of a hum trimmer in VH4 amps as sometimes the trimmer gets fried.

If you are not sure which resistor I am referring to you can email me the 2nd photo and I will mark it for you.

Kind regards,

Jens

The left resistor should be 47 Ohm, the right one 100 Ohm.
No replacement in this case.

 

[Jens Kruse]

I could not see from the photo that the left resistor is already set at 47 Ohm.

Jens

 

[i.ak]

Beautiful! ...but can't see any trimmers.

A bit OT, but kind of related as it's a tubey thing as well: I have had significant issues with similar 6550/KT88 tube-retainers as you have. They seem to cut to the glass in some occasions.

To prevent the retainers from cutting into the glass you can buy a rubber ring (see image below).

Jens

Good advice. Might be worth checking the heat resistance capabilities of the rubber tube though before installing.
I have moved back to EL34s, but definitely helpful to someone having same problem.