MikeC Sixty-One SRT

[Bg]

Happy to do a cream/gold 1x12 cab for a hand shandy.






time to crash the server again.

[MikeC]

What are you going to do with the prototype?

Dunno - get it going properly!

[robthemac]

What are you going to do with the prototype?

Dunno - get it going properly!

If it needs a bit of a run-in at any stage, you know where to find me.

[MikeC]

What are you going to do with the prototype?

Dunno - get it going properly!

If it needs a bit of a run-in at any stage, you know where to find me.

I will absolutely bear that in mind - I'd already thought of JB for the task as he's already "out" (as an AC10 lover)! By the time I'm finished I'll be way too close to the project so much better to get some independent opinions/suggestions.

[MikeC]

V Nice! 7189 tubes?

Yep (EL84s for the rest of us)!

[MikeC]

Happy to do a cream/gold 1x12 cab for a hand shandy.






time to crash the server again.

The quick and the dead (I'm too slow on this bargain)!

[RectifiedAmps]

Nice build! How's the clean sound on it? Doug and Pat mention in one of their shows that the SRT doesn't really 'do' clean tones, which always puzzled me.

[MikeC]

Nice build! How's the clean sound on it? Doug and Pat mention in one of their shows that the SRT doesn't really 'do' clean tones, which always puzzled me.

I'm still sorting a few things so can't really report back on tones yet. The AC10 SRU schematic that I built from (the one I sent you) causes B+ to be too high and also puts too much voltage on the EL84 plates (so they end up biased too hot at about 115% of MPD with the 130 ohm cathode resistor). So I've modified the power supply so that the entire amp is fed via the 1K ohm R36 resistor (I need a 10W one which I've ordered, currently using a 5W one so I can't run it too long) - this gets the voltages about right and cools the EL84 bias. Clean power has dropped to about 10W rms from a previously measured 12W+ rms. I haven't cranked it yet (apart from on the scope with a signal generator into a dummy load). I also want to try changing the cathode bypass cap to 100uF (currently 25uF) to get a bit more bass response. Actually there's quite a few things to work on.

[RectifiedAmps]

How much do you need to drop the B+ by? If you're feeding the OT CT thru the 1K resistor you'll end up with a lot of sag and things will get spongy. Depending on how much voltage you need to drop, resistors between the PT and the rectifier anodes might be a better option, or a resistor between the PT CT and ground. If resistors are still giving too much sag, then a high wattage zener diode is a better option. Using it between the PT CT and ground will prevent noise due to increased ripple.

[MikeC]

I understand your points and welcome your input.
I need to drop the plate V and screen grid voltages by about 40-50V to match the AC10 SRU schematic.

Notice on the attached "other" AC10 SRT schematic (OS-71), the rectifier's output all goes thru R18 (1K ohm 10W resistor). I did some calculations from readings with my amp at full tilt and that resistor is dissipating 9.6W (so the 10W rating on the OS-71 schematic would seem correct).

My PT voltages match the AC10 SRU schematic (the one I sent you).
But in that schematic it appears that the OT primary is dropping 95V. Doubt it Harry!
Putting the rectifier's output all thru R18 (1K ohm 10W resistor) gets me close.
But without getting to see the guts of an actual Vox AC10 SRT I won't know for sure how they've done it.

I played thru the amp this morning (cranked) and there isn't an audible sag issue.
The journey continues!

SRT OS-71.jpg (39.05 KiB) Viewed 414 times

[jeremyb]

What does sag sound like? Can you view it on a scope?

[MikeC]

What does sag sound like? Can you view it on a scope?

"Sag", like "bloom" - hit a big loud note or chord with the amp cranked and the power supply can't instantly keep up so there is a kind-of delay in the volume and fullness. Tube rectifiers exhibit a voltage drop as their output current increases and it's especially noticeable (also to to the ear) when asked for a sudden increase. It's desirable for many (not all). I can't think of a practical way to see it on the scope.

[Slowy]

Dunno - get it going properly!

If it needs a bit of a run-in at any stage, you know where to find me.

I will absolutely bear that in mind - I'd already thought of JB for the task as he's already "out" (as an AC10 lover)! By the time I'm finished I'll be way too close to the project so much better to get some independent opinions/suggestions.

Really interested in this too Mike. It swims in the same pond as my Jansen, VHT and Harmony. Or at least it appears to.

[Slowy]

What does sag sound like? Can you view it on a scope?

"Sag", like "bloom" - hit a big loud note or chord with the amp cranked and the power supply can't instantly keep up so there is a kind-of delay in the volume and fullness. Tube rectifiers exhibit a voltage drop as their output current increases and it's especially noticeable (also to to the ear) when asked for a sudden increase. It's desirable for many (not all). I can't think of a practical way to see it on the scope.

Kind of the opposite of a big solid state amp which punches you in face when you pick a string.
Sag is beautiful until you have a massive heap of notes and very little time to play them all.

[jeremyb]

What does sag sound like? Can you view it on a scope?

"Sag", like "bloom" - hit a big loud note or chord with the amp cranked and the power supply can't instantly keep up so there is a kind-of delay in the volume and fullness. Tube rectifiers exhibit a voltage drop as their output current increases and it's especially noticeable (also to to the ear) when asked for a sudden increase. It's desirable for many (not all). I can't think of a practical way to see it on the scope.

Great explanation, so it would appear to kinda swell into the note or chord?