Question for electronics gurus

Factual Questions
1

Here’s the story, as concise as I can make it. It probably takes very specific knowledge of this equipment, but maybe the problems will ring a bell with one of you

I have an integrated amp for my stereo with vaccuum tubes.

Well, one of the tubes was not lighting up so I sent it in for repairs. They retubed it, reset the bias, and burned it in for about a week and sent it to me. No probs.

I ran it for a few days, then one day the tubes didn’t warm up (the fuse blew during warm up). I replaced a fuse and everything was fine. Ran it a couple days, blew a fuse again during warm up.

Hmmmm. . .the repair guy sent me new tubes. Before they arrived, I was listening to music one night and the tubes just quit. The unit still has power. It has little fans that blow on the tubes and the fans were still running.

I plugged in the 4 new tubes, and nothing.

I called the repair guy and he said something like, “hmmmm. . .might be the power source. What’s the voltage in the line?”

“Dunno”

“I set the bias for our voltage which is 127.”

I don’t know the implications of that. He said line voltages can run from between about 110 and 130.

Can any provide possible “trouble shoots” for this type of behavior?

2

Maybe, you could try telling us what it is :slight_smile:

Do you know whether it’s the heater circuit or the HT that is giving out? My experience of valve amps is mostly based on guitar amps, but I have never had more than one tube die at the same time. If tubes are actual being killed rather than just not getting any juice then something is very wrong.

By the “voltage in the line” do you mean the mains supply voltage?

If I can Google up anything useful on biasing valve/tube amps I’ll be back.

BTW you’ve made me all :frowning: now that I sold my old Quad 22.

3

This is the amp. There’s a PDF file there (but the link is html) that shows how to make bias adjustments.

I do mean the main supply voltage. (I think)

I don’t know whether it’s the heater circuit or the HT that’s giving out. I"m not even sure what you mean with the question. There are 4 tubes in it.

4

Apologies for raising more questions than answers. Tubes are powered by two separate circuits, one heats the elements to scare electrons off the cathodes the other supplies the actual power as a high voltage. The circuits may have a separate fuses, are there any internal fuses?

From this site

Hmm… might be the bias? I couldn’t see the bias adjustment page do you have another link?

5

If the bias is set wrong, then the devices will draw a lot of current if the amplifier is class B.

If it’s class A then all you get is assymetric amplification at higher outputs and hence distortion.

I don’t really see how supply voltage could make too much differance, there would be probably at least around a 10% tolerance, and anyway the amp would, to some exent, be self regulating, after all the power supply and smoothing should take care of it, as long as you have set it to the normal household range in your country.

Valve heater voltages are generally quite low so I can’t see that this is going to be an issue, but its also not that hard to check, its just a simple D.C measurement

I would suspect the driver phase splitter area to be most likely to be a problem, so that the quiescant current from one side of the amp is very high.

You can get a problem in class Bs where both output devices are turned on, when of course only one at a time should be conducting, depending upon the signal direction.

A good giveaway would be to measure the quiescent current and compare it with the specs, then I would get on with checking the dc voltages around the driver part of the amp.

I expect that you’ll find its drawing much more than the specs allow when there is no input signal.

Things like open circuit (more likely)and short circuit(less likely) resistors in the driver, and bias section will be most suspect, though if there are electrolytic capacitors about (especially tantalums) then these may well be troublesome.

Pity you don’t have a link to the circuit diagram.

6

This is the direct link. It’s a pdf. If you give it a minute, it should load. It just hung my browser for a few seconds.

7

I don’t think the company likes to have the schematic out there.

See, when I sent it in the first time, they replaced all 4 sockets, all 4 tubes and set the bias “correctly”. It was working and sounding great.

Also, it’s a class A.

Also, Small Clanger when it was working, the “plates” aren’t heating up, but the filaments are. The problem I had before I sent it in originally, the plates WERE heating up.

8

How about a new amp? They’ve recently gone solid state. :stuck_out_tongue:

9

Well, I know.

I’m sending this one back for more repairs (which will now represent about $75 in shipping charges alone for like a $600 amp). :frowning:

The same company makes real nice looking solid-state integrated, and also real nice looking pre-amp/amp combos within my price range.

I really like the sound of this amp, though. I’ve had a newer Sony that I didn’t like, which I sold to someone and an old Marantz receiver that is basically a piece of crap.

I might get the guy to give me a discount if I buy through him. Really don’t know if I want to give tubes another shot, or go SS though.

10

As a producer/engineer I wholeheartedly agree that the tube sound is sweet.

11

To the OP, measuring your incoming AC voltage is easy.

Go to RadioShack, and buy an inexpensive digital multimeter.

Set for AC Voltage, and place a probe in each of the slots in the outlet you’re likely to use this amplifier in. Read the voltage on the screen. Should be betwen 110-130V AC.

Report that number to the tech doing the work, so that they can set the bias correctly.

Note that your incoming voltage is provided by your power company, and you may see fluctuations on the line. What’s there today, may not be there when you check next, but it should be close.

Good luck.

-Butler