unloaded voltage readings

[TomMcNally]

Jeff -

OK - I didn't realize this was the amp you were working on, obviously it's far from a standard ST-70.

Let me ask once again (now that you say the output tubes are flickering) ... have you hooked up speakers to this amp ? What do you hear ? I am still thinking it's oscillating.

The voltages aren't that far off (in percentage) - but if those are CChong's ST-70 boards - they are only 1/2 loaded. Or are they the MK-III boards ?

... tom

[jeffdavison]

No speakers i can use at the moment and my preamp is back at BAT getting repaired and will be without for a while longer.

CChong board is for the ST70

I've been in conversation with him about this and he wants to send be some diodes and resistors to swap on the cap board for different values.

JD

Jeff -

OK - I didn't realize this was the amp you were working on, obviously it's far from a standard ST-70.

Let me ask once again (now that you say the output tubes are flickering) ... have you hooked up speakers to this amp ? What do you hear ? I am still thinking it's oscillating.

The voltages aren't that far off (in percentage) - but if those are CChong's ST-70 boards - they are only 1/2 loaded. Or are they the MK-III boards ?

... tom

[TomMcNally]

There is a 50-50 chance that the amp is oscillating due to the feedback being backward ... without listening to it, it's hard to say what's going on. here is something you can do ... are the load resistors getting warm ? They should be stone cold with a shorted input.

[jeffdavison]

They do get very warm but not hot.

Feed bak taken from the 16 ohm output tap to where it should gon on the driver board. The line is routed around the the inside of the chassis away from all other internal wiring.

JD

There is a 50-50 chance that the amp is oscillating due to the feedback being backward ... without listening to it, it's hard to say what's going on. here is something you can do ... are the load resistors getting warm ? They should be stone cold with a shorted input.

[TomMcNally]

Jeff -

The load resistors should be stone cold ... with the inputs shorted, the amplifier isn't putting out any power. What I think is happening is that the feedback tap is reversed. If you swap the transformer primary leads (ONLY - not the secondary) on each channel, your load resistors will be cold (as they should be) and your tubes will stop flickering - and your amp should work just fine.

... tom

[jeffdavison]

not exactly sure what you mean by reveresed. I have followed the ouput transformer diagram to the letter and have taken the feedback tap from the 16 ohm output (in the case of the Edcor I'm using) it's green and it feeds precisely to the turret that Mapletree specifies for the feed baack signal. This has been double triple checked. So I am confused .
Screen leads go to screens, plate leads go to plates.


My brain is as fried as those screen resistors, This was supposed to be fun. I wish I could just send it to someone at this point as the confusion and frustration is beginning to be not worth the effort. I have been going over and over and over this thing and all the wiring checks out as required.

JD

Jeff -

The load resistors should be stone cold ... with the inputs shorted, the amplifier isn't putting out any power. What I think is happening is that the feedback tap is reversed. If you swap the transformer primary leads (ONLY - not the secondary) on each channel, your load resistors will be cold (as they should be) and your tubes will stop flickering - and your amp should work just fine.

... tom

[erichayes]

Hi Jeff,

Before you let minor setbacks discourage you (and believe me, they are minor), disconnect the feedback leads from the output transformer secondaries . . . just let 'em hang loose. Turn the amp on with the inputs shorted and the outputs terminated with your resistors.

As Tom has stated, the load resistors should run at ambient temperature at this point. Because there is no feedback, negative or positive, being applied globally, it is effectively out of the picture.

If the resistors stay at ambient temp., the next step is to swap the plate and screen leads from one output tube to the other on both channels. Even though you are convinced that your wiring is correct, transformers do get wound and/or labelled incorrectly--and electrons don't give a rat's ass who's right or wrong.

If the resistors still heat up, then there's a wiring problem elsewhere. Keep this in mind: the problem is occurring in both channels. The odds of an identical component spontaneously failing in both channels are astronomical. Therefore, the problem is in something common to both channels--power supply--which is highly unlikely, or a wiring error that's been duplicated in both channels.

I'm going to be out of town until Wednesday evening, so I won't be able to follow this saga to, I hope, its conclusion. Good luck, Jeff

[Shannon Parks]

My bet is that you have a feedback oscillation causing your problems. Though I did lose a $15 bet on the Bears last night.... Yellow_Light_Colorz_PDT_29

First, disconnect your negative feedback from the speaker taps and power it up again (once you've replace those resistors). I bet she'll be stable. Heck, pour a beer and listen to it for a bit if you'd like. Then post some links to some schematics, grab an oscope and square wave generator, and we'll see what we can do for you. Even if you don't have an oscope, we can probably at least drop the feedback 6dB or so and get it stable enough to not blow resistors.

[kheper]

jeffdavison check to see if the board
is grounded to the chassis. My star
ground was not properly soldered
to the board's star ground jack.
The load resistors smoked, until
I fixed it. The load resistors
connected to 'c' terminals
arced to the closest screw
on the chassis.

[jeffdavison]

no grounding issues.

jd

jeffdavison check to see if the board
is grounded to the chassis. My star
ground was not properly soldered
to the board's star ground jack.
The load resistors smoked, until
I fixed it. The load resistors
connected to 'c' terminals
arced to the closest screw
on the chassis.

[kheper]

What eirchayes wrote:

'If the resistors stay at ambient temp., the next step is to swap the plate and screen leads from one output tube to the other on both channels. Even though you are convinced that your wiring is correct, transformers do get wound and/or labelled incorrectly--and electrons don't give a rat's ass who's right or wrong.'

The transformer primaries are hooked-up
phase inverted. If the amp is squealing,
the inverted wiring would explain the
problem.

I think you can swap the drive lines from
the board to the output tubes as well
to correct this.

[jeffdavison]

Mega thanks for the mega help from Tom to help walk me thru some things. Turns out the main problem was phasing, but not transformer. It was the grid leads being reversed. Even tho I proofed and proofed and checked and double checked, I finally discovereda typo difference between and early instruction revision and a late rev. of the schematic of the driver.

Soooo,, oscilation is taken care of, load resistors bone cold with the global feedback from the 16 ohm tap in place and no flickering tubes.

Have a bias issue to work out as the values in the C.Chong cap board gives me from .7 to 2.25 v bias per tube WAAAAYYY to high. He's sending me some altered value components to get the range needed for the 6550's. His board was designed for a standard ST70 running EL34s and one board per amp. I'm using two boards and 6550's, so his thinking is to supply the values that he uses for his MK3 cap boards..



Also thanks the Mike (MeSherm) for straightening me out on the proper way to put in the triode diodes. seems the way I thought he told me to install them actually blocked the plates and all the current was
runing through the screens. can you spell barbeque?

Now that I have the oscillation sorted, and hopefully will have the bias sorted out, I just may have a go at the diodes again.

I'm sort of glad things fried now. With all the help here and digging deeper into the "how and why" I am alot less intimidated and understand the amp more than I would have if everything went perfectly the first time. Im a lot more dangerous now.

Jeff

[erichayes]

Hi All,

Just FYI, if you had reversed the transformer leads, you would have accomplished the same thing. Phasing is phasing, and can be corrected using either input or output reversal on the output tubes--but not both. Sometimes, when you've got all your transformer leads cut to exact length, it's easier and less messy to swap the coupling caps. It's also half as many wires to move and resolder.

[EWBrown]

IIRC, the lines from the driver board to the EL34 grids had to be "crossed" as compared to the original ST70 (7199) boards, due t the different LTP design of the phase splitters. Easise to swap two connections than four OPT primary wores...

/ed B in NH

[jeffdavison]

Thanks to all... She's a making a nice a musica now! Nice a and a stable. I can save on my gas bill this winter,

Even without having everything in the system broken in... :)


Have a small issue with some hum... from both channels. Going from a 3 prong to 2 prong with ground not connected to house ac.. same same.

Ground loop? Each channel has it's own star ground point ( true dual mono everything) and a connection from each to the ground point of the Corcom filter / IEC outlet / switch module.

Things to try? ideas?

JD