Neg Feedback / oscillation Questions

[TomMcNally]

in another message - Shannon said:

swap the signal wires going to the EL34's like an 'X' to swap the phases around so that you can get negative feedback instead of positive feedback from the 16 ohm tap.

That may be your cure.

[WA4SWJ]

Hmmmmm,

Interesting. I've built an ST-70 clone with Shannon's board and it works great. I did not use special capacitors - just Orange drops - but it works and sounds great. It would be interesting to learn what you find out. Mine worked first time right out of the box.

I did put a solid state rectifier board in mine (forgot where i got it Yellow_Light_Colorz_PDT_10 ) because my GZ34 arced over. No more problems.

Good luck with yours. I don't think the circuit is unstable. I would think something else is wrong but it's hard to offer advice without looking at it.

Regards,

[Shannon Parks]

Hi madranker,

We Have found the circurit very interesting, but do not understand the use of only neg feedback from the yellow wires of the output trannies...

I'm a little confused by this comment. Do you mean the lack of the screen bypass cap joining the overall feedback as in the original Dynaco circuit? Yes, that has been omitted in my design. There is also local negative feedback is used in V1.

This amp goes into oscillation when the volume is turned up and there is basically no bass.

There is something definitely wrong - as an American I love my bass. Yellow_Light_Colorz_PDT_19 Could you please take DC voltages per the voltage sheet and I'll take a look at that for you.

and looking up the LM334Z Specs and swaping it around because your printing on the top of the board and bottom are mirrored..( don't match)... Good thing my solder sucker suks Properly... Yellow_Light_Colorz_PDT_09

The LM334Z is an active device, so notice that pin 1 will go into the same hole whether placed from the top or bottom of the PCB. So the silkscreen is correct on both sides. Possibly your LM334Zs are damaged now? That certainly would cause you all types of headaches and funky behavior.

Shannon

[Shannon Parks]

Could you please take DC voltages per the voltage sheet and I'll take a look at that for you.

Just curious, is there a revised manual with test voltages? The downloadable version doesn't have them.

Hi Wodgy,

I've been emailing the voltage chart file with the Mouser BOM. It'll be included in the manual eventually, I just wanted to double check it first.

I'll try and make all the files downloadable (including a printable manual and the BOM files).

Shannon

[Shannon Parks]

Mostly all i get to this point is uncross the wires J1,J2,J22,J23 so mostly what I'm hearing is it makes no difference if you follow the design schematics or not ? Yellow_Light_Colorz_PDT_10

Hi madranker,

AFAIK, the schematics, silkscreen and manual are indeed correct. The design is a robust modified Mullard circuit and is not a radical departure from the original operation. Old or new outputs shouldn't cause you a problem as I use a very conservative feedback network. All development testing was done with a circa 1961 Dynaco ST70.

I'd be happy to look at your board free of charge. We'll get you humming along in no time.

Shannon

[mesherm]

I installed Shannon's driver in one of my ST-70s although I used 0.1 and 0.22 mf Solen caps instead of orange drops. I also installed a Cyrus Chong cap/bias board at the same time. The chassis and PT are original, the OPTs are Triode Dynaclones. It powered up and ran fine first time.

[leadtower]

I re-soldered and ohmed out the PS board; re-placed it and the Amp fired up perfectly. I am so much a novice that I bareley understand this stuff but have very good luck with Shannon's boards; both BB and ST 70 which is playing as we speak.
I would examine the PS and all wiring first before shipping.