VHT Special 6 out of volume

[wole]

Hi everyone, it's my first time posting here and also the first time trying to repair an amp, so please excuse me if my questions are trivial

Some days ago I was playing thru my VHT Special 6 combo and I noticed something strange in the sound: it appeared very low in volume, like it had a blanket on it, like it lost all its sparkle.
For this reason I compared it with a Special 6 head I have (both are non-ultra version and run the same type of tubes: 6v6gt and 12ax7) and the difference was clear! Comparing them on the same speaker the head was almost twice as loud as the combo!
It's clear something went bad in the combo.. Could it be because a friend of mine the day before used the combo for more than an hour at max volume (yes, at 10/10) in boost mode?

At first I thought of the tubes: they were not the problem because I switched them between the head and the combo and the difference was still the same (head almost twice the volume).

So I opened up the combo to search for some burned electrical component but everything seemed alright at first sight.
At home I had just a voltmeter so I checked just the bias resistors (the 470ohm and the 5K ohm ones) and they were fine.
I've read that the problem may be caused by a bad capacitor: for this reason I'm buying a capacitor tester and I'm going to test all capacitors and resistors in the circuit.

I know I'm being quite vague and it's not easy to help me out like this, but I have a question.
Can you think of something else (except capacitors and resistors) that could have caused the problem of the loss of volume?
Since it's my first time repairing an amp I would greatly benefit from knowing which are the most probable causes of the problem.

Thank you very much and sorry again for my inexperience

[Dave H]

Schematic

[nickb]

I think you'd be wasting your money on a cap tester. A DVM is the first thing to reach for and use it to measure the voltages on each cathode, grid 1, grid 2 and plate pin. Troubleshoot. Don't spend time randomly testing or replacing components.

[wole]

I think you'd be wasting your money on a cap tester. A DVM is the first thing to reach for and use it to measure the voltages on each cathode, grid 1, grid 2 and plate pin. Troubleshoot. Don't spend time randomly testing or replacing components.

Thank you. But how will I be able to test if a capacitor went bad without a cap tester?
Also, by "cathode" you mean cathode resistor?
What do you mean by measuring the voltages on grid 1,2 and plate pin? Should I check voltages across what?
Finally what do you mean by "troubleshoot"? Is there a standard procedure?

Again I'm sorry, I realize my knowledge is very limited..

[nickb]

Thank you. But how will I be able to test if a capacitor went bad without a cap tester?
Also, by "cathode" you mean cathode resistor?
What do you mean by measuring the voltages on grid 1,2 and plate pin? Should I check voltages across what?
Finally what do you mean by "troubleshoot"? Is there a standard procedure?

Again I'm sorry, I realize my knowledge is very limited..

No problem. We all had to start from ground zero sometime.

By measuring voltages we can check the basic operating conditions. This will check the power supply, the tubes, quite a few resistors, connections and even some capacitors using a fairly small number of measurements.

It may be a capacitor, but the chances are you won't need a cap tester to find it. We should be able to narrow things down so that you test one or two by substitution. New caps are very, very cheap compared to a good cap tester. Having another identical amp to compare with may help too.

First, you need to find a reference point to clip your black lead on to. In many amps this is the chassis but certainly the negative end of C111. With that in place you take you red lead and start probing.

With no guitar plugged in and with the amp on, probe and report the DC voltages at the following points (search on the Internet if you not sure how to identify the pins).
V101A pins 1 (plate), 2 (grid) , 3 (cathode)
V101B pins 6 (plate), 7 (grid) , 8 (cathode)

And for both low and high power positions of the switch do.
V103 pins 3 (plate), 5 (grid) , 8 (cathode), 4 (grid 2)

Also measure the power supply volts with the switch in 'high' at points marked 'A', "B", "C"

Good luck and most of all be careful - use one hand - keep the other in your pocket, just in case.

[wole]

No problem. We all had to start from ground zero sometime.

By measuring voltages we can check the basic operating conditions. This will check the power supply, the tubes, quite a few resistors, connections and even some capacitors using a fairly small number of measurements.

It may be a capacitor, but the chances are you won't need a cap tester to find it. We should be able to narrow things down so that you test one or two by substitution. New caps are very, very cheap compared to a good cap tester. Having another identical amp to compare with may help too.

First, you need to find a reference point to clip your black lead on to. In many amps this is the chassis but certainly the negative end of C111. With that in place you take you red lead and start probing.

With no guitar plugged in and with the amp on, probe and report the DC voltages at the following points (search on the Internet if you not sure how to identify the pins).
V101A pins 1 (plate), 2 (grid) , 3 (cathode)
V101B pins 6 (plate), 7 (grid) , 8 (cathode)

And for both low and high power positions of the switch do.
V103 pins 3 (plate), 5 (grid) , 8 (cathode), 4 (grid 2)

Also measure the power supply volts with the switch in 'high' at points marked 'A', "B", "C"

Good luck and most of all be careful - use one hand - keep the other in your pocket, just in case.

Thank you again.
By the way the schematic must be of the ultra version because the regular special 6 only has one 12ax7.
I'm sorry but I still have some questions:
Once I've found out the tension values at the points you mentioned what should I do? Confront them with the tension values of the well working head special 6?
Also, once I've found out a suspicious tension value how can I determine which component is broken?

Thank you very much

[nickb]

Thank you again.
By the way the schematic must be of the ultra version because the regular special 6 only has one 12ax7.
I'm sorry but I still have some questions:
Once I've found out the tension values at the points you mentioned what should I do? Confront them with the tension values of the well working head special 6?
Also, once I've found out a suspicious tension value how can I determine which component is broken?

Thank you very much

There are two triodes inside the 12AX7 glass envelope. The reference is "V101" and each triode section is called "a" and "b". So, in the schematic they are marked as V101a and V101b.

As you measure the voltages, write them into a nice neat table (please) and post the results back on this thread. There are loads of people here who will be able to interpret them for you and give you the next steps.

[wole]

There are two triodes inside the 12AX7 glass envelope. The reference is "V101" and each triode section is called "a" and "b". So, in the schematic they are marked as V101a and V101b.

As you measure the voltages, write them into a nice neat table (please) and post the results back on this thread. There are loads of people here who will be able to interpret them for you and give you the next steps.

Thank you, now it's clear to me!
I will do it in a couple of days since I'm not home right now, but then I will post the values in a nice table!

[Dave H]

I tried to upload the official schematic from the VHT web site but it's a pdf and I got the dreaded single pixel black screen. I found a handy online converter so here it is as a png. It's a starter for ten. There are voltages marked on TP1 to TP4 and btw welcome to the forum wole

[g1]

Here's the pdf version:

[wole]

Thank you!!

[Dave H]

Here's the pdf version:

That works fine. It's the pdf I tried to upload. How come I only got a single pixel?

[J M Fahey]

Suggest you download and read (many times ) Jack Darr's classic book "Repair your guitar amplifier" where the introduction is available as a .pdf at some sites.
Of course, if you like it try to get the full printed book (often available at Amazon and other online bookstores) .
VERY clear and to the point explanations.
FWIW I started with it 45 years ago

[Mick Bailey]

You mention buying a cap tester - if you post a link we can give you some better advice, but in general most practical circuits (especially tube amps) operate capacitors at voltages much higher than those used by test units. So a capacitor can test OK, but give a fault under operating conditions.

The actual value of a capacitor is rarely of concern in audio circuits, and I can't recall ever changing a capacitor just because the value had changed. More likely a capacitor will short out, go open circuit, or leak DC. There's another factor which affects capacitors; ESR - Equivalent Series Resistance. A capacitor not only has capacitance, it has resistance and inductance, too. A faulty capacitor can increase in resistance to the extent where this can seriously affect its ability to pass AC. Whilst I find an ESR meter to be really useful, a DMM will show the symptoms of high ESR in circuit without the need to actually measure a value.

[wole]

Thank you very much everybody, really great advices!