Vibro Champ with EL34 (builder past away before finished) advice needed

[Kees]

Dear Readers,

Last week i bought a tube amp from a friend. This amp is finished but not yet tested and the builder has passed away. I do have the schematic. I think it basically resembles a vibro champ, but equipped with an el34.

I think the amp is very neatly built with top quality parts, but according to the schematic a few parts are missing. When I look at the Vibro Champ I have the feeling that the missing parts are not a problem, (but I'm not an expert on tube amps). Can someone confirm that the missing parts are not a problem?

Also a resistor of 82K (on the power tube pin 5) has been used instead of the 1.5 K according to the schematic. This does not seem correct to me, and is perhaps a mistake from the builder. Please some advice.

As a last question, where can I attach the indicator lamp of the vibro switch, that has not been connected yet.


I hope you can advise me. 


Greetings
Kees





https://i.postimg.cc/MZBbRdmC/buizenversterker-schema-01-opm-eng.png

[sluckey]

I made some notes on your schematic. I would not connect the lamp to the tremolo circuit. But if you must then just connect as shown on the schematic. The series resistor should be a high value. Try 220K.

Good luck. Let us know how it turns out.

[sluckey]

Also a resistor of 82K (on the power tube pin 5) has been used instead of the 1.5 K according to the schematic.

I just realized you are using EL34. 5K6 is more commonly used on pin 5.

[Kees]

Hello Sluckey,

Thanks a lot for watching and replying.

A few more comments/questions so that I can finish te amp:

- The Dept pot-meter is already connected to ground. So that will be fine. But if I want to fine-tune the dept pot, by adding the missing resistor, do I have to use a special resistor for that? There are several different types in my amplifier?

- Where is the lamp in the schematic? The strange N symbol in the tremlo section or the X? Why do you advice not to connect? (PS; de X is not a cut wire)

- I want to move the controls to the front of the amp (for cabinet making). Actually that is no more than a quarter turn. Is there anything I should think about / avoid?

- the resistor in the powersupply part is 5K instead of 8K2 (see drawing). I assume I need to check the voltage of A+, B+ and C+ after switching on the amp and thereby check the resistance?

- What is your opinion of the amp? Is it build correctly? Can I / do you advise to adjust the amp locally?

https://i.postimg.cc/jdMLP5tm/buizenversterker-schema-01-opm-eng-02.png








A very happy man. I'm going to finish that amp.
Kees

[sluckey]

- The Dept pot-meter is already connected to ground. So that will be fine. But if I want to fine-tune the dept pot, by adding the missing resistor, do I have to use a special resistor for that? There are several different types in my amplifier?

Let it be! There's nothing to do here.

- Where is the lamp in the schematic? The strange N symbol in the tremlo section or the X? Why do you advice not to connect? (PS; de X is not a cut wire)

The lamp is the N2 symbol on the schematic. All it does is provides a lamp that flashes with the beat of the tremolo speed. If you just have to have it, get the proper neon bulb for that lamp holder. Don't forget the series resistor I mentioned earlier. The lamp is probably the same as in the power indicator. If you are lucky there will be a proper lamp already in the lamp holder. If there is, take a pic and post it here.

Just ignore the X. That's for schematic purposes only. It simply means the two C- points are not connected to the same physical point, even though they are electrically connected.

- I want to move the controls to the front of the amp (for cabinet making). Actually that is no more than a quarter turn. Is there anything I should think about / avoid?

Terrible idea. This is a high quality build using high quality components. Even the sockets and terminal strips are high quality items. It would be a shame to hack it up!

- the resistor in the powersupply part is 5K instead of 8K2 (see drawing). I assume I need to check the voltage of A+, B+ and C+ after switching on the amp and thereby check the resistance?

Let it be. Nothing to do here.

- What is your opinion of the amp? Is it build correctly? Can I / do you advise to adjust the amp locally?

I'm very impressed with the workmanship and quality. The builder obviously knew what he was doing and took great pride in his work. Please don't hack it up. If you do, please don't post any hacked pictures.

A very happy man. I'm going to finish that amp.

Looks like all you need to do is plug it in and test it.

[Kees]

Sluckey,

Also a resistor of 82K (on the power tube pin 5) has been used instead of the 1.5 K according to the schematic. I just realized you are using EL34. 5K6 is more commonly used on pin 5.

Could it be that the 82K was placed deliberately? Can it hurt to turn on the amplifier with the 82K?

I Will keep The Controls as the are. And if the 82K is ok, fire it up comming days (of changement it to 5K6 and also fire it up). I Will let hou know.

[sluckey]

82K is not harmful. It's just unusual. After seeing these nice pics I'm convinced that the builder purposely put that value on the grid. I just don't know why.

[sluckey]

V3 socket... There must be a jumper between pin 1 and pin 8 for EL34 tube. Not shown on the schematic and I can't see it in the pics. Please verify.

[Kees]

V3 socket... There must be a jumper between pin 1 and pin 8 for EL34 tube. Not shown on the schematic and I can't see it in the pics. Please verify.

It was already done, see pics.



free photo hosting

82K is not harmful. It's just unusual. After seeing these nice pics I'm convinced that the builder purposely put that value on the grid. I just don't know why.

I assume the 82K may be doing something to my volume and nothing to the sound. is that right?

Kees

[sluckey]

82K may be used to reduce some high end treble, make the amp a bit more mellow sounding.

[Kees]

He did wrote on the schematic

Moonlight tone.

So perheps intended.

[Platefire]

I agree with sluckey, the lead dress and circuit installation is just outstanding. I keep looking at those threaded mounting brackets and the way he has got those located, I can't help but think he had something special in mind to mount this chassis to. Did he leave any indication on that? Platefire

[Kees]

I agree with sluckey, the lead dress and circuit installation is just outstanding. I keep looking at those threaded mounting brackets and the way he has got those located, I can't help but think he had something special in mind to mount this chassis to. Did he leave any indication on that? Platefire

Hello platefire,
I never met the deceased man. I saw the amp around my local craiglist and the quality immediately caught my eye. The asking price of €150 seemed reasonable to me (although it was a gamble). According to his wife (who I vaguely happened to know) he was always trying to improving (his) tube amps.
What I also understand is that his intention was to mount the amplifier in a RCA 400 16mm projector box, but the whole with speaker didn't fit.

[Kees]

Hello Sluckey,

I turned on the amp and the positive news is that the amp works. But:
- the potentiometer Tone does not change the sound
- the Vibro does not work (not even with a jumper)
- the two potentiometers of the Vibro do not make any difference in sound (not even with a jumper)
- when I turn the depth potentiometer fully open, the sound goes off.
- there is little gain when I turn up the input potentiometer (which works by the way)
- there is a strong hum from 3/4 open master potentiometer

The deceased man's wife tested the amp, but didn't know where to place the tubes (and I don't know if a speaker was connected. The fuses (1 in the connection cable box and 1 extra on the control panel) were then blown.

I have a feeling that 1 or more of the preamp tubes 6SL7 are not working. Is that also your idea? Would you also start there ?

PS: I don't understand why there are 2 fuses in the amp.
- 1 in the main input 630 mA
- 1 additional 500 ma.
I'll check to see if they're on different veins

Greetings Kees

[sluckey]

Tubes are always suspect and easy to change. Measure pin voltages for all tubes. Also measure voltage on all four filter caps.

I'm not familiar with that tone circuit. Doesn't look like it would be very effective.

That vibrato circuit does not need any jumper to operate. Measure tube voltages. If V2-2 seems erratic on a DMM the oscillator is probably working. If steady, the oscillator is not working.

Don't be concerned about two line fuses. I suspect that after installing the front panel fuse he decided to use a fused IEC socket also. Just let it be.

[Kees]

Hello Sluckey and others,

Christmas and family are gone, so I got back to look at the amp.

I changed the tubes. That made a lot of difference in the sound of the hum. A lot less and only audible from 80% opening of both volume pots. But the rest of the problems remained.

I also played on the amp for a while, but the sound is actually lousy. Even my wife thought it sounded shrill, it actually hurts her and my ears. (PS: maybe this is also due to the speaker used, a Celestion G12p-80 line6, and just in a cardboard box for tryout)

I measured the voltages. They can be seen at the link. Can you or someone else get something out of this? and give me some advice? (I love the fender blues amps tone)

https://postimg.cc/tsJhgwRh



if I temporarily remove the wire from the dept potentiometer to pin 6 of V1B, then the vibro is switched off, right? Or is signal still generated via B+ and B-? Is that a way to fine-tune the base of the amp?



Greetings Kees

[pdf64]

Speakers will make whatever you put through them sound awful, unless they’re mounted in a cab.

[Kees]

Speakers will make whatever you put through them sound awful, unless they’re mounted in a cab.

Perhaps I have to borrow a speaker from one of my friends and see what will happens then.

[Kees]

I made a mistake, I probably had to measure The heater voltage in AC.