In my Vox AC15H1TV resistors R32, R24, R33, R21 different from the schematic !?!

[hruki]

Hi,

Can someone please help me out with the following issue:

Here is the story: in the beginning I thought that what I experienced was related to the power tubes. My amp went dead, and the first thing to do is probably to check the tubes. When I changed the two EL84 and the fuse, amp was working again. Then I find myself with 4 pairs of "non working" EL84 in less then a year of light amp use

Even for an Vox amp, "the tube eater" as people say it is too much. So I opened the amp to do some research, and found five resistors with different value then in the schematic. Here it is: http://www.blueguitar.org/new/schem/..._handwired.pdf

In my Vox the R24 and R32 measure 3,3kΩ (in the schematic 1,5kΩ), R21 and R33 are 470Ω (in the schematic 100Ω). I will add one more, with less dramatic difference: R28 should be 130Ω, in my amp the value is 148Ω.

For the end I'll just add that recently I had an opportunity to check those tubes I thought dead and found that all of them are still working.

My technical knowledge is limited so any kind of input will be welcome Is it on purpose that those values are changed? Is it an erreur? If so what are the consequences? Should I change those values back to what I see in schematic? ...

Thank you all in advance for your help!

Hrvoje

[Jazz P Bass]

Welcome to the site.

I would imagine that Vox would know why they changed the resistors.

R24/R32 are the grid leak resistors.
No big deal that they are larger.

R21/R33 are the screen resistors.
Again, no big deal.

R 28 is the cathode resistor which sets the bias of the tubes.

It sounds to me as if they where trying to 'tame' the EL84s with the values that you have in your amp.


Note: in the future, as a sign of respect to other members, kindly indicate where a specific component is on which schematic.

[Enzo]

I suspect that the HTVH schematic might differ from the H1TV schematic in some details. R24,32 are grid stoppers and won't hurt the tubes being larger. Likewise R21,33 are screen grid resistors, and will actually be a little easier on the tubes at 470 than at 100 ohms. If R28 measures 148 ohms, when you get that reading, exchange your meter probes end for end - reverse the leads. Does the same reading result? Or does it now read a little low? being a few ohms high should not cause problems.

Notice that all the fuses in this amp are slow blow types - time delay fuses. Are your fuses slow blow types? You didn;t mention which fuse was blowing. Also, as you report, the amp isn;t eating tubes, it is eating fuses.

[hruki]

Welcome to the site.

I would imagine that Vox would know why they changed the resistors.

R24/R32 are the grid leak resistors.
No big deal that they are larger.

R21/R33 are the screen resistors.
Again, no big deal.

R 28 is the cathode resistor which sets the bias of the tubes.

It sounds to me as if they where trying to 'tame' the EL84s with the values that you have in your amp.


Note: in the future, as a sign of respect to other members, kindly indicate where a specific component is on which schematic.

Thank you Jazz P Bass for your welcome, thank you both for help! I got the point about need to indicate where the components are, I have a lot to learn. Now, to answer to Enzo, I will post what I mesure at R28 with reversed leads as soon as I get back home, should be in couple of days. I do have slow blow fuses (the one that was blowing all the time is FS1 (1st schematic G2), but couple of times the FS4 also (1st schematic H2). And you're right, the amp seem to be the fuse blower :-) What I don't really follow is why in my first experiments by changing only the fuse, I just blew another, but when i put the new pair of EL84 with the third fuse, everything was back in normal. And that was the case about three times, that same story. That is why I begin to think that I'm blowing tubes...

[tubeswell]

Check that the tube socket pins are sufficiently tensioned

[Steve A.]

If it continues to blow fuses please post voltages at power tube pins and the other power supply nodes (the ones that supply voltage to the other tubes.)
Also, is the amp driven hard when it blows the fuse(s)?

Steve Ahola

P.S. Why would you get different readings of cathode resistor when reversing test leads, Enzo?

[Enzo]

Because of a small residual voltage sitting in the parallel cap.

Your ohm meter sends a small current through the circuit. ANy residual voltage in the circuit can do one of two things. It might buck your meter current, and so make the reading look higher. Or it might add to the current, lowering the resistance reading. In fact, and it takes very little, enough voltage in the circuit and you get a "negative resistance" reading on the meter.

So if I measure a resistor, it ought to measure the same either way, but if I measure it both ways and the readings differ, I can infer a little voltage is clouding the issue.

He COULD be facing that, making his 130 ohm read 148. Or it could be that the resistor has indeed risen in value. Or we didn;t look close enough and they installed a 150 to begin with.

[Mick Bailey]

I always check the hum-balance resistors if the heater fuse (FS1) blows. Sometimes HT can get onto the heater circuit and increase the current draw through the fuse, but this will mostly cause one or both of the resistors to go open. Are R9 and R10 reading good?

You could always have a bad rectifier tube.

BTW, what is the marked value for R28?

[tubeswell]

Mind you, modern production EL84s ain't what they used to be - especially the ones from Mike Matthews factory. I've had new ones redplate straight out of the box with the bias voltage set correctly and everything. Try looking for mil spec Russian EL84Ms or 7189s

[hruki]

Ok, I'm back in town! To start with, I feels good to be surrounded with all of you guys: people with knowledge, experience and willing to help!

I checked most of things that all of you suggested, the things I did not verify now are good ideas for me or anyone who will in the future have similar problem, not knowing where to start from.

So to answer you and to give some info to others, if and when in need, there it goes:

The Vox has send me the repair manual for AC15H1TV. Schematic is the same as one from the head version that we all consulted.

The resistors measured show the same thing as their color code suggest (R28's marked velue is 150Ω, I measured 148Ω). So it probably is, as Jazz P Bass suggested, the Vox modification that was not reflected in the schematic. By the way, all resistors' measure OK (R9 and and R10 too). Enzo, I reversed the leads to measure the R28 with the same result both ways.

Now the part for Mick and Tubewell: first time I only tested the JJ EZ81 from the non working amp. It was way out of balance, so I replaced it with a vintage Mazda, which I bought verified and working. Now I decided to go further and cheque if "working" Mazda is really working. The result: I replaced a bad rectifier tube with another bad rectifier tube. So yes, happy ending with a thing to remember: you never know what you have without a tube tester (if you think your tubes are ok, think again, 'cause you dont know).

Thank you all once again, keep on rocking :-)

[Steve A.]

Mind you, modern production EL84s ain't what they used to be - especially the ones from Mike Matthews factory. I've had new ones redplate straight out of the box with the bias voltage set correctly and everything. Try looking for mil spec Russian EL84Ms or 7189s

One caveat: sometimes older versions of EL84s use a tube pin no longer in use and board designers run traces through that pin assuming it is okay. More specific details to follow (anomaly was found in Crate V18.)

Steve Ahola

[TimmyP1955]

Make sure that R24 and R32 are right on the tube sockets, as opposed to being on the circuit board and connected to the sockets with lengths of wire. With some tubes (JJ for sure, possibly others) this is essential for stable operation.

[The Dude]

I'm not saying it is/was the root problem, but it wouldn't hurt to measure the voltage across R28 and calculate tube current. It's not uncommon to have to change cathode resistors in these to get the tubes to reasonable dissipation.