The Amp Garage

I think the leakage current, even with the e-caps either exceeding their voltage rating, or reverse biased, would have been too low to cause short-term damage. Certainly changing the e-caps to 100nF/100V would avoid any cap stress. Those two caps to gnd provide some transient current path arising fr...

That 'optional' dc heater portion in the Solo 100 schematic does seem a bit confusing. The CT of the heater winding sits at elevated dc. If the 'option' was installed then C39 and C40 would sit nominally at +/- a dc level from the elevated dc, except for the influence of the C41/42 gnd point. Imho, ...

Maybe also worth a note on what to do when the cathode (or anode) currents are not the same, or the amp is a quad. Eg. at what % higher/lower current on one side would it be reasonable to adjust the target and why.

Perhaps also an indication of nominal period to make a repeat measurement.

All my articles are 'living', and I don't include a revision list as I typically end up doing many many updates over a few years. The date on an article is the latest 'update' date. I always seem to find something new to add 'just around the corner'. I recall chasing down RG's articles, and emailing...

Can you clarify if both OPT primary winding sections (Anode to B+, and B+ to Anode) were shorted? It's not simple to appreciate why that happened along with GZ34 failure and both HT and mains fuse - and my first thought would be both sides of the output stage were conducting heavily, but unsure if t...

How did you confirm the OPT was 'blown' - was a short circuit or open circuit on a half-primary winding? How did you calculate 18W dissipation ? For example, 50mA idle through each EL84 would be 200mA through the common cathode resistor - if that resistor was 50R then that is 10Vdc at idle. If Plate...

Holistically/practically speaking WRT a given circuit, is monitoring the preamp plate voltages the most obvious tell-tale that too much DC is getting by or are there more indicators? To add to Martin's comments, your above comment about monitoring a plate voltage is sort of correct, with the provis...

Yeh I'd also recommend looking further at the use of MOV's in lieu of zeners as Martin suggests, as they should 'blur' the transition between parallel arms and avoid any stepping characteristic. Whilst zeners likely have a tighter tolerance on operating knee, Zeners and MOV's each have their own SOA...

Imho it's worthwhile to also appreciate the current loop that forms from the main reservoir capacitor terminals, out to the output transformer primary winding B+ terminal, and back through the output stage valves to a cathode end junction that returns to the reservoir capacitor negative. That 'loop'...

The feedback stage has a 470pF bypass on its 220k anode load, which would start rolling off gain above say 1kHz. The 2.2nF introduces a corner frequency at 22kHz, which is quite low. That seems to be too low to use to improve stability margin for one common compensation scheme where the network chan...

Yes tonally the amp may oscillate wildly and grossly depending on the speaker and leads used - its called unstable operation.

Thanks for taking the time to describe your experience :) Did your recent mods change the output stage in anyway that may have influenced the cathode fuse to nearly blow ? Did you have an input signal applied to the amp when you turned it on without a speaker load ? Normally the cathodes wouldn't ex...

One of the basic tests I do for any transformer I want to use, but don't know its origin, is for insulation resistance at say 1kVdc between windings, and between windings and core. That test is a quick form of 'hi-pot' test that would be done by a manufacturer prior to sale, and similar to a sparky'...

Did you match them into pairs at least for that price?

Given something like what you'd want would be quite niche, you could check with Scott to see if he has/would make a jig and gain unit for you. Ronald's uTracer has a distortion assessment tool which may get you the information you are after, but not for noise comparisons, and is a commercial partly ...