4x6L6 OT and PT

[daddario]

Hello!
The schematic is a Mesa Double Rec. (http://www.freeinfosociety.com/elect...iew.php?id=507)

So far I have two questions:
OT Impendance: If I have understanded correctly, then Raa should be 2.8kohm. It's quite hard to find such a transformer. Most 100W and 120W have a Raa = 1.9kohm.

PT power: How much should the coil for the anode voltage bear?
The datasheet says, that for two 6L6's in class AB1 tetrode with fixed bias and Vp= 450V(460V on the Mesa) the Ip max is 210mA. So for four tubes, that is 420mA + 40mA for the preamp tubes = 460mA. Seems a little bit high?

Thanks!

[pdf64]

OT - where have you got the Raa of 2.8kohm figure from? I can't see it on the schematic. If correct and that's what you want, you would probably have to get one from Mesa (they will probably want the serial number of an actual dual rectifier which you can pretend to be repairing).
PT - with PTs, it depends how good you want the regulation to be, that is how much the voltage drops as the current demand rises. And then do you specify full demand as maximum unclipped output, or when the output is maximum square wave? It's easiest to buy a PT from a reputable manufacturer which is made for a similar amp, such as a fender twin reverb in this case. Or get one from Mesa as above. Peter.

[daddario]

The OT:
On a Svetlana 6L6 datasheet it says:
Class AB1 tetrode connection, fixed bias
Vp: 450 v
Vscr: 400 v
Grid bias or res. -37 v
Ip idle: 116 mA
Ip max: 210
Iscr idle: 5.6 mA
Iscr max: 22 mA
Load: 5600 ohms
Pout: 55
THD (approx) 1.8%

So, for 4 tubes the Raa should be 2800 Ohms.

PT: Hmm. What kind on PT did the Dual Rec. use? How much load could it bear until the voltage drop became over normal?

[pdf64]

OT - So are you asking about svetlana's test rig, or a mesa dual rectifier?
If a dual rectifier, then the OT is likely to be close to the music industry standard type.
Music industry tube amplifiers put out more power, at the expense of more THD, than the datasheet type. They don't have the screen supply regulated to 50V below the plate supply.
Are you wanting to build a copy of a dual rectifier, or repair one which has bad transformers? If so, a transformer set (that is power transformer and output transformer) suitable for a fender twin reverb would likely be fine. Peter.

[daddario]

OT:
Hammond Replacement for Dual Showman, Twin Reverb output transformer (125A29A & 022889). 2000ohm CT / 4ohm 100W
But it doesn't exactly suit me, beacuse it only has a 4 Ohm secondary.
But a transformer with 1.9 kOhm Raa should be ok? For tube amp a load with a little smaller impendance is ok.

For the PT I still need to know, how much current does the Mesa draw from the anode voltage coil. Fender doesn't have a suitable transformer. Those with a 5V coil for the recifier filament are 50W versions.

Choke: A 4H, 100 Ohm, 90mA should be the right one?

[Gregg]

The OT:
On a Svetlana 6L6 datasheet it says:
Class AB1 tetrode connection, fixed bias
Vp: 450 v
Vscr: 400 v
Grid bias or res. -37 v
Ip idle: 116 mA
Ip max: 210
Iscr idle: 5.6 mA
Iscr max: 22 mA
Load: 5600 ohms
Pout: 55
THD (approx) 1.8%

So, for 4 tubes the Raa should be 2800 Ohms.

Svetlana and others impedance figures found in datasheets are for linear, HiFi operation. Forget about them if we're talking guitar amps.
All 100-120W guitar OTs have 1,7k-2k. If you're into Hammond get a replacement OT for JCM800 or 900 and you won't be wrong. You'll have 4, 8 and 16 Ohm taps as well.
The choke is OK for 6L6s.
PT should be ~400mA for the HV winding.

[daddario]

Thank you both.
I'm going to buy the OT and the choke from Hammond, but the PT will be a Dual Rect. transformer from TAD.

[daddario]

I forgot one thing. LDR's!
Does anyone know what the light and dark resistance should be? Or what type of optocouplers they use?

[pdf64]

Not sure but OPT-2 maybe the kind of thing
https://taweber.powweb.com/store/chansword.htm

[daddario]

Hmm. The light resistance seems kind of high. Again I'm not sure.
I was considering this: R10: 8.3 ohm, R01: 85 kohm, R05: 255 kohm, Vmax: 200 V, Pmax: 200 mohm, Ton: 35 msec, Toff : 28 msec

[daddario]

Okay! According to this: http://w3.enternet.hu/zwerner/tx4.jpg the Triaxis uses VTL5C9 vactrols. I don't think the Dual Rect. should be any diffirent. The VTL5C9 datasheet can be seen here: http://www.tubeampdoctor.com/images/.../SW5C9DATA.jpg

[pdf64]

I think that either the OPT2 or the 5C9 from TAD would be fine. They are probably the same part, but from TAD is more expensive, but then there's delivery and taxes to factor in. As you'll need so many, it's worth checking on the total cost from both. Peter.