MikeC Sixty-One SRT

[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.

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

Yes - it can sound lovely. But not so good for big metal power chords...

[jeremyb]

"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?

Yes - it can sound lovely. But not so good for big metal power chords...

Would this be why amps like Mesa's dual and triple rectifiers have more than one?

[MikeC]

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

Yes - it can sound lovely. But not so good for big metal power chords...

Would this be why amps like Mesa's dual and triple rectifiers have more than one?

I'd say yes. Multiple rectifiers provide the power providing capability when it's needed/wanted and also gives the user choices for more "spongy" thru to "solid" by which rectifier(s) they select. NOTE: I'm presuming there are switchable choices as I've never owned one.

[RectifiedAmps]

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

If it sounds good then keep it, but 1k (1.23k with the internal resistance of the EZ81) is a lot of series resistance for the current drawn by the EL84s. Even if it's not sagging excessively, it could be responsible for the lack of bass response you mentioned.

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.

Having the 1k before the OT seems to be what Vox intended. The later AC10 710 schematic also shows the same thing and the EF86-equipped 60's AC10s and AC15s had 100R resistors between the EL84 anodes and the OT primaries, so it seems like something Vox frequently did in the 60's.

The schematic also shows the EL84 screens at 300V and the anodes at 295V - that's a bad idea for valve life and I'd recommend some screen resistors to be safe.

[olegmcnoleg]

The schematic also shows the EL84 screens at 300V and the anodes at 295V - that's a bad idea for valve life and I'd recommend some screen resistors to be safe.

This explains a lot

[RectifiedAmps]

The schematic also shows the EL84 screens at 300V and the anodes at 295V - that's a bad idea for valve life and I'd recommend some screen resistors to be safe.

This explains a lot

Ha! Ok, more details: when the screens are at a higher voltage than the anode, they 'steal' extra current away from the anode. Screens aren't rated for as much current as anodes, so if they draw too much it stresses them and can lead to them failing, which usually means a short circuit. Adding some screen resistors drops the voltage on the screens closer to that at the anode, plus it provides some current limiting if the screens start to draw too much.

[olegmcnoleg]

The schematic also shows the EL84 screens at 300V and the anodes at 295V - that's a bad idea for valve life and I'd recommend some screen resistors to be safe.

This explains a lot

Ha! Ok, more details: when the screens are at a higher voltage than the anode, they 'steal' extra current away from the anode. Screens aren't rated for as much current as anodes, so if they draw too much it stresses them and can lead to them failing, which usually means a short circuit. Adding some screen resistors drops the voltage on the screens closer to that at the anode, plus it provides some current limiting if the screens start to draw too much.

Thanks for the extra detail. Fascinating stuff.

[MikeC]

One good thing with old Vox designs is that the Screen & Anode voltages don't exceed the maximums stated in the specs of the EL84. Unlike Fender Blackface DR's which grossly exceed the voltage specs of the 6V6 (apart from the JJ 6V6S which is more like a 6L6). In my DR builds I always install 1K/3W screen grid resistors and only use JJ 6VSs. But I will put some in the SRT - what values do you recommend?

[MikeC]

I've managed to improve the reverb. On the reverb recovery triode I bypassed the 33K resistor on V3 pin 8 (the cathode) with a 22uF cap. The gain went up (according to the calculator) and I could hear the improved amount of reverb. I then plugged in a 12AT7 instead of the 12AU7 and the gain improved by about 2.5 times (according to the calculator). Now the reverb pot doesn't need to be set at maximum. Progress!

[codedog]

Any updates? Are we there yet?

[MikeC]

Any updates? Are we there yet?

Patience my boy! So, I've gone back to the 12AU7 as the reverb driver/recovery tube because I managed to get some more out of the driver side with an added cathode bypass capacitor. This makes the reverb strong enough with an AU7 but I'm now trying to improve the reverb tone (it's a bit "crashy" to me). I'm also trying different speakers as the tone from Doug's SRT is so smooth - here's the YouTube clip https://www.youtube.com/watch?v=alBO_yEn5o4. They have mic'd their cab for the recording so it probably sounds different in the room. My amp is still sounding a little harsh to my ear and I am yet to change any component values from those on the schematic. And I don't have an LP with PAFs like they use - the closest I've got is a Riviera with mini hums. That's got to be part of it too. Is there anyone nearby (I'm in Red Beach) that can lend me an LP with PAFs? And I'd love to try a 25W Greenback (M magnet) as well

[MikeC]

Update:- reverb tone sorted at last, did someone just say "drip"? I ended up using a single tube design from an 1962 Ampeg ReverberRocket which has a 12DW7 at its heart. This tube is 1/2 12AU7 (driver) and 1/2 12AX7 (recovery). The reverb sounds surprisingly good to me. And there's plenty of it on tap, so much so I reduced it. Progress! Only problem is I'm spending all my time on it.

[bender]

Update:- reverb tone sorted at last, did someone just say "drip"? I ended up using a single tube design from an 1962 Ampeg ReverberRocket which has a 12DW7 at its heart. This tube is 1/2 12AU7 (driver) and 1/2 12AX7 (recovery). The reverb sounds surprisingly good to me. And there's plenty of it on tap, so much so I reduced it. Progress! Only problem is I'm spending all my time on it.

Oooh! I used to have a Reverberocket clone, and it had the best sounding verb I’ve heard in an amp.

A Vox with that verb? Colour me interested!

[Marshmallow]

Hey how are you finding working with turret boards?

[MikeC]

Hey how are you finding working with turret boards?

They are much better than eyelet boards IMHO. And I can build boards with turrets - I have the kit for them.