JTM45 Build with various output tubes and GZ34/Solid State Rectifier

[Spencerleehorton]

HI All,

attempting to build a JTM45 without the GZ34 and relace with a diode bridge.
I have an old mains transformer from an amp which has the 6.3vac for the heaters for the 3 x 12AX7's and 2 x 6L6's.
It has several other windings 2 x190vac with CT, a 60vac winding and 2 x 10vac with CT winding.

I was wanting to get the B+ 450vdc from the 2 x 190vac = 380vac but then wanting to reduce the AC using a big resistor to 320vac.
this is where i may come unstuck, but hopefully you guys can help.
I have calcualted that 5 x 40ma for the plate current draw from the tubes on B+ giving 200ma, dropping 60v at 200ma i would need a 300R resistor, not quite sure of the power but im thinking its either 5W or 10W resistor?

Am i correct please?

 

[Spencerleehorton]

Well I work the power of the resistor needed is 12watts?

0.2 x 0.2 x 300 = 12 watts

Did I get that right?

 

[Spencerleehorton]

Or could I put 2 x 5W 150 ohm resistors in series?

 

[Matador]

60V @ 0.2A = 12W. Would need more like 20W if you wanted it to live a long time. Two, 150ohm, 10W resistors, would work, one in each leg of the secondary before the rectifier diodes. Would be permanently cooking so long as the amp is on, so heatsinks seem warranted (or chassis-mount power resistors).

 

[Spencerleehorton]

Or I could get a 25W heatsinked 300 ohm, that should run a little cooler?

I just wondered whether I’d calculated the current draw correctly?

 

[Matador]

You need to experiment. 70mA for the output tube pair, and maybe 30mA for the others. So somewhere between 100mA and 200mA in practice in total.

Like I said, I would split into two resistors and make a balanced filter with the first cap.

 

[Tubetec]

Could a full wave rectifier with centre tapped secondary get you closer to target volts ?

12ax7's only draw a few mA per section max ,

Im not sure why they felt the need to add a standby in an amp with a tube rectifier ,
Hot switching tube rectifiers is not reccomended .
In any case you will need to implement a proper standby arrangement .
You can use a DPST switch on the secondary or an SPST in the centre tap to ground .

 

[Spencerleehorton]

could you explain how full wave rectifier with CT will get me close to 320v with 2 x 190vac please?

i might go with what matador suggest and get 2 x 150R 25W and put them before the diode bridge

 

[zzzzz]

Excuse the messy photo, this is a trace from 7 years ago. Some related frame of reference.

The earliest Traynor YGA-1 amps (basically a JTM45 clone with tremolo) from the mid 1960s used 7027A power tubes not unlike a 6L6GC (would otherwise be EL34/6CA7). I have one and it was loaded with Canadian General Electric 6L6GC’s when I got it and it was great.

Someone had converted this to use a solid state rectifier at one point, just four diode bridge, but then someone else converted it back to tube.

These amps have Hammond transformers and Hammond still keeps their line of Traynor spec xformers in production to this day. If you send them an email, they probably still have data on the original power xformers too.

https://www.tubesandmore.com/products/transformer-hammond-output-replacement-yorkville-traynor

 

[Spencerleehorton]

Im Sorry but how does this help me get my mains transformer from 2 x 190v with CT or 380vac down to 320v?
I’ve been pretty clear in the title saying I’m NOT going to use a tube rectifier and use a diode bridge?

 

[danfrank]

Why don’t you go the esoteric route? Your transformer has all the required windings to do this. Just make sure that the windings have required current capacity (at least 200ma) and isolation:

Put the two 190vac windings in series to get 380vac. Now put the 60vac winding in ANTIPHASE with the 380vac windings. In phase, you will get a total of 440vac so you will want to switch the 60vac leads so they are antiphase and your total windings voltage will be 320vac.
As long as the 60vac winding has good voltage isolation from the other transformer windings, it will work well.
A much more elegant solution than using big power resistors which will cause the B+ to fluctuate wildly because of the varying current requirements of the actual amp in operation.
If you’re using fixed bias, use the two 10vac windings in series in a voltage doubler configuration for the bias supply.

 

[absrec]

Im Sorry but how does this help me get my mains transformer from 2 x 190v with CT or 380vac down to 320v?
I’ve been pretty clear in the title saying I’m NOT going to use a tube rectifier and use a diode bridge?

If I'm understanding your post correctly, the transformer you have would give you a rectified voltage of around 270 VDC.

190 * 1.414 = 269

This will end up giving you a working B+ of around 250. I don't think you can wire them in series for a few reasons. For one, you'll only get half the rated current of the transformer. The other thing is your heaters (usually) have to reference the same ground.

If you want 320V DC, you need an AC transformer rated for around 500VCT or 250-0-250. After rectification and filtering, you'll end up in the range you're looking for. That is assuming a fairly typical layout as far as how many gain stages you have, etc. A lot of factors to consider.

 

[merlin]

Put the two 190vac windings in series to get 380vac. Now put the 60vac winding in ANTIPHASE with the 380vac windings and your total windings voltage will be 320vac.

Provided the 60V winding can handle the current, this is the ideal solution. But was that 60V winding intended for a low current bias supply? Can you see the wire thicknesses of the windings to compare?

 

[CJ]

GZ34 would sound better.
And it would drop your plate voltage. Just need a 5 vac xfmr.

If voltage still a bit high you plug in a 5U4.

 

[Matador]

But was that 60V winding intended for a low current bias supply?

That's what I was thinking, since a bias tap is one of the most common configurations/uses for these low(er) voltage windings.

 

[Spencerleehorton]

The mains transformer is from a line 6 spider hd100 amp

https://www.proaudioservice.co.uk/product/line6-spider-mains-transformer-hd100/

Line 6 11-30-0029

Line 6 Guitar Amp Power Transformer

220 Volt power transformer for Line 6 Spider Valve guitar amps.

XFMR 220/240VAC 170/60/9.8/6.3 SV A15-1/

I also have the output transformer from this amp as well

 

[Tubetec]

I think having the resistors in series would effect the sound under clipping conditions , still worth a try ,
Mains transformer secondary winding resistance , resistor and output transformer primary resistance form a divider.

I dont like the idea of phasing ac windings to cancel voltage ,
as well as unwanted heating of the transformer and all the previous caveats , like wire diameter ,insulation and placement to adjacent windings .

Why not just live with the transformer you have and adjust the series resistances by ear?

Theres no doubt about it if you want the 70's hard rock sound you want silicon diodes for that extra edge ,
if you want to sound like Peter Green era Mac tube rectifier is the way to go ,

I wouldnt be bothered building a 50W or 100w guitar amp anymore
A pair of EL84's is more than enough volume for any situation nowadays where the amp is driven .

 

[Cqwet Dbdfte]

If the tubes can take the extra plate voltage, don't sweat it.
However, the bias supply would need to be adjusted to a higher negative value to keep plate current in spec. Small resistors in the output tube would facilitate current measurement, like 5 ohm or so in each
Driving screen grids from a current source is not my favorite, a regulated DC supply is better, can easily be ramped for slow start like a GZ34.
Adding series resistance to the rectifiers is OK, but less output power.
A CL90 or similar AC mains current limiting resistor can take care of inrush current if you have large caps. (I have a bank of photo flash caps,).
Keeping the bias voltage correct will assure not overheating the output tubes.

 

[SoundBlaster]

You could also go with a choke input power supply.. Rule of thumb is that DC out = approx 90% of the AC voltage feeding it. You can fudge that up by using a very small film cap before the choke. The small cap provides a little ripple reduction helping out the averaging/integration down by the choke. You will have a very quiet power supply.. Choke input generally works better with a class A amp as the current through the choke is fixed.

 

[Spencerleehorton]

well i think my best bet it to go with the original design and keep true to it, so i'll speak to demeter and get some transformers and choke ordered, all 3 will only cost £180.
i can have the diodes or tube rectifier switch, this seems the best option sound wise.