West Grande mod

[flintrock]

The West is up and running, sounds very good. Having a problem with the L.O.U.D. switch. With it off signals runs as usual through the tone controls. Switch it up to loud and it is much louder, it just grounds the circuit from the mid pot to ground. Played a minute on this setting then blew a fuse.

[flintrock]

Any reason that would suck more power?

[flintrock]

Another somewhat strange thing is the use of two 470uf 50v caps in the optional 12v wiring for v1. The schem calls for 25uf 15v. Quite a jump, but I have heard this is common for hi fi designs. Taking some voltages to get it sorted out.

[flintrock]

Also should I have a ground wire from the bus wire to ac grounding point? Its a black painted steel chassis. Both boards ground only at that bus wire.

[martin manning]

The LOUD switch sounds like a tone stack lift, which would un-ground the mid pot when it is engaged. That will increase the drive to the following stages, but shouldn't blow a fuse.

For the optional DC supply for the 12AX7, I think the only ground for the filament circuit should be V1's pin 9 (no CT ground). The two caps could be even bigger if you like.

Do you have pictures of the bus grounding?

[flintrock]

That is correct, the 12ax7 heater circuit grounds through pin 9 only. I can try to get some pics up later, but that bus does ground at the input jacks. I had it running a bit last night on LOUD with no problems but only for a few minutes with no instrument plugged in. Going to give it another run through tonight with a bass to see if it still has the issue.

[flintrock]

I confirmed the loud switch does lift the ground to the tone controls.

[flintrock]

Having a problem with one tube red plating, they are a new pair of Valve Art 6550's. Swapped tubes around and the red plating followed the tube. Assuming a bad tube, so I tried a Sovtek 6550WE I had new in a box, only just the one though. Now I have 29ma on one tube and 89ma on the other with equal -54 bias voltage on each tube. 560 plate voltage! Thinking new I need a closer matched set of tubes or this voltage is too high?

[flintrock]

Question, I measured 555 on the screens, should I run KT88's in place of the 6550's? I don't think 6550's can handle that, I see max screen on those is 400v.

[Leo_Gnardo]

Now I have 29ma on one tube and 89ma on the other with equal -54 bias voltage on each tube. 560 plate voltage! Thinking new I need a closer matched set of tubes or this voltage is too high?

Yes these are way out of balance. And KT88 would be a good choice. I've used JJ and had success with them. You may want to look into reducing the screen grid voltage anyway.

[flintrock]

So I guess I'll add some screen resistors, 1k 5w? Ok to use pins 4&6? And would this drop my screen voltage to acceptable 400v? I'm running 555 at the pin 4 now. And should it be set up for 6550's or KT88'S? The schematic calls for 6550's but I assume the older tubes were more hardy back then. These new ones can't handle it. Lets get this rare amp going!

[flintrock]

Now that I think about it, I have to use KT88'S.

[martin manning]

Do you know what the OT primary impedance is? Is it running UL?

[flintrock]

It does supply power to the screens (Pin 4). I don't remember how to measure the ot but I'll post that as soon as I figure that out.

[flintrock]

OT 5020 @ 8ohms, 4160@4ohms, 4690@16ohms is what I came out with. Sounds like this amp would be a good candidate for a zener diode voltage mod, I'm running 565 volts on the plates with screens just about the same. I have some JJ KT88'S coming just hope they fit.