The lack of service response from the seller notwithstanding, I'll second the vta board. Readily available tubes and Bob Latinos design make for a reliable combo. Pretty solid instructions as well.
Help with modded Dynaco ST-70?
- Thread starter Raccoon1400
- Start date
Lavane
Lunatic Member
I did the same with mine. I also got rid of the 5AR4 rectifier and use a WZ-34 SS rectifier that mimics the voltage from of the 5AR4. The picrure below is before I installed the WZ-34.
Attachments
Love those digital displays, I need to get a pair for mine.
Mine would like a set, but then I'd have to re-home the bias pots that currently live there. No idea where to put them, which is why they are there now. I suppose maybe I could swap the existing test points out for 4 of the small chassis mount pots I like to use, but then I'd have to re-design the bias/balance system I have now for individual pots.
Lavane
Lunatic Member
Dynakit sells bias trimmers built into a pair of sockets for the front.
There is an adjustment screw in the keyway hole in the center.
Lavane
Lunatic Member
They are a little bright. It doesn't bother me, but some have added #35 car window tint to put on the front of the plexiglass. You can find scraps at any place that does car window tinting. The meter kits don't come with diodes, so they can flicker a bit. Again, it isn't enough to bother me. I don't sit there staring at them while listening to music.
mine is using a pair of concentric pots in place of the front sockets, front knob is bias, rear is balance. If I put a pair of meters in the front sockets those pots lose their home. I can't put pots in the original rear location, holes are way oversized and the cap board lives under there now. About the only place I think they might be able to live is where the pin jacks now live near each tube for setting the bias. With 4 displays and a pot next to each tube it just seems to make more sense to have it be individual bias rather than bias/balance.
You could put in one meter and a rotary switch to swap between tubes. Much less real estate.
still would need to do something with the pots, and then there is a switch to find a home for.
I own cars with zero instrumentation that are an "ignorance is bliss" situation but I can't quite bring myself to do that with my amps.
at the pace I re-visit projects the most likely thing that will happen with this amp is absolutely nothing and 10 years from now its going to be exactly like it is now. Hopefully still working as well as it does now.
Lavane
Lunatic Member
Rogue does something similar. One bias meter and four switches under a small plate up top. There are two thumb screws holding the plate in place.
The muzishare X& does this as well, but I believe it uses 2 meters. They display VU but can be switched to tube 1 and 2 bias on that channel.
whoaru99
Epic Member
I check the dipstick every now and then, or as needed if something seems off.
Yeah I own a 2012 Honda civic that didn't have a temp gauge and I had to add one to it
To me a "hot light" is like waiting to see a tube red plate to know something is wrong.
I've installed these and like them a lot.
Raccoon1400
Super Member
I ordered the exact same part as the tubes that were working in this thing and they still draw the voltage way down! 2 new 6H23 and put them in the first spots in the signal path.
Something seems bad with the math here.
A tube that's drawing 270V across a 30K resistor, is drawing 9ma.
That's a LOT for any preamp tube.
There's got to be something weird about the cathode circuit, or input grid circuit, that's biasing the tube that hard.
I'd check out the feedback loop and cathode resistor wiring, to see if something is inadvertently too low of a composite resistance (combination of cathode and feedback circuit components resulting in too low of a parallel resistance) in the cathode circuit of the input tubes.
The other possibility, is something that is somehow leaking or otherwise impressing a positive DC voltage on the grids of the input tubes. I'd check to see if the grid ground resistors are within the workable range (less than 1 megaohm). If not, the tube could be drifting in bias on its own.
Regards,
Gordon.
A tube that's drawing 270V across a 30K resistor, is drawing 9ma.
That's a LOT for any preamp tube.
There's got to be something weird about the cathode circuit, or input grid circuit, that's biasing the tube that hard.
I'd check out the feedback loop and cathode resistor wiring, to see if something is inadvertently too low of a composite resistance (combination of cathode and feedback circuit components resulting in too low of a parallel resistance) in the cathode circuit of the input tubes.
The other possibility, is something that is somehow leaking or otherwise impressing a positive DC voltage on the grids of the input tubes. I'd check to see if the grid ground resistors are within the workable range (less than 1 megaohm). If not, the tube could be drifting in bias on its own.
Regards,
Gordon.