correct PT for a 18 watter

[Davidscott]

Hi,
I'm new to this forum. But not new to building tube amps. I'm an engineer, but still have not grasped the power concept of tube amps.
I am building a hybrid 18 watt based off a schematic found from Mhuss, called TRex.
But I am adding a SS opamp driven spring reverb, and may use 6V6 instead of EL84. So basically a 6G3 but with a TMB tone stack.
On all the Marshall inspired 18W combos I have looked at, none of them describe the power requirements for this amp.
I have a Hammond 290AX, it is 650vct @ 70mA (website states 80mA), 6.3V @ 2A, and 5V @2A. I am also using a SS diode rectifier.
Does anyone know if this PT will provide enough current for this style of amp?

Thanks.

[JMPGuitars]

That PT won't be sufficient. Compare to the 290PAZ for typical 18W specs.

https://www.hammfg.com/files/parts/pdf/290PAZ.pdf

[AleS]

Secondary voltage is quite high, though you could bring that down with diodes (see JMP's signature links) or a droppy valve rectifier, but 70/80mA is way too low for an 18w, you're looking at 120 at the very least (the 290PAZ posted handles up to 140).

[Davidscott]

thanks for the info. guess I will have to flip the bill for a new PT.
Oh and in thinking this over some, I need a +-15vdc supply as well. I have a small +-15V PCBA that will provide this to my opamps, however, it needs 30Vac (i meant 15-0-15) VAC tranny. I could buy a small tranny for this.... But If I can find a main PT with a additional winding (perhaps bias voltage) I could use that. Or... what if I tapped into the 290paz two 6.3v and 6.3v lines to give me 12.6VAC, once I rectified that, I could get a +-12vdc line which might work fine for my opamps.
thoughts?

[Bieworm]

thanks for the info. guess I will have to flip the bill for a new PT.
Oh and in thinking this over some, I need a +-15vdc supply as well. I have a small +-15V PCBA that will provide this to my opamps, however, it needs 30-0-30 VAC tranny. I could buy a small tranny for this.... But If I can find a main PT with a additional winding (perhaps bias voltage) I could use that. Or... what if I tapped into the 290paz two 6.3v and 6.3v lines to give me 12.6VAC, once I rectified that, I could get a +-12vdc line which might work fine for my opamps.
thoughts?

Maybe look for a toroidal. Those have more different windings often. I bet you TubeTown has some
Or add an additional small transformer, just for that opamp application. That will probably be cheap

[Davidscott]

I'm still confused. in comparing xfrmers.
You see I want to build a Marshall style amp (derived from the Mojo tone 6G3) that uses 6V6 in the power section, and then If I want to later, I can convert to EL84 using the yellow jacket socket converters. But I have a dilemma, the 18watter (AKA marshall) style would use a PT 290PAZ that has 290V-0-290V, but a 6G3 uses 290BX that uses a 330V-0-330V, which is a lot higher than the 18 watter. So, should I use the higher voltage PT? Or the lower voltage? Or does it even matter? I don't care if its "18 watts" or not, it could be 15, or 20, I don't really care. I am after the sound, not the watt power.
please help, I don't want to buy the wrong xfrmr.

[JMPGuitars]

You can safely run 6V6 tubes in EL84 voltage territory; the reverse is not true.

[Bieworm]

FWIW you can perfectly use a 290-0-290V secondary for F*nder type amp builds. I advise you to use a SS/diode rectifier for maximum B+. I've done that and it yields about 365VDC B+, plenty for the F*nder amps, especially tweed or brownface amps. If you're aiming for 6G3 territory the lower B+ will only be beneficial when it comes to tone and dynamics.
Another path is to use a transformer with about 240 or 250V secondary. That way you can aim for a B+ around 300 to 320VDC. Then you can use 9 pin EL90 tubes to simulate the 6V6 vibe or rewire to EL84 and rebias to run those. The EL90 is a very good substitute for a 6V6GT, but the plate voltage has to be lower. On the tube datasheet max Va is about 275V, but you can run them at 300-320V easily. Even if they wear more quickly, they are dirt cheap and many NOS good quality is available. I buy Svetlana Winged =C= EL90 for about 4€/pc
A 3rd option is that you use the Hammond 290CAX. This one has a 550VCT and a 630VCT secondary. With some planning and using a rotary switch or DPDT switch you can get 2 different B+ values. One for EL84 (550VCT) and one for 6V6 (630VCT)

[Davidscott]

thanks for the suggestions! I might go with that 290PAZ at 290-o-290. and yes I am planning on a SS rectifier. maybe I am trying to make it too fancy. I already have 2 other 6V6 P-P style amps. so this will be my first EL84 style. so perhaps I will just stick with that. The chassis I am using has tube openings the size of 6V6, so I will need to make an adapter plate to cover them...

FWIW you can perfectly use a 290-0-290V secondary for F*nder type amp builds. I advise you to use a SS/diode rectifier for maximum B+. I've done that and it yields about 365VDC B+, plenty for the F*nder amps, especially tweed or brownface amps. If you're aiming for 6G3 territory the lower B+ will only be beneficial when it comes to tone and dynamics.
Another path is to use a transformer with about 240 or 250V secondary. That way you can aim for a B+ around 300 to 320VDC. Then you can use 9 pin EL90 tubes to simulate the 6V6 vibe or rewire to EL84 and rebias to run those. The EL90 is a very good substitute for a 6V6GT, but the plate voltage has to be lower. On the tube datasheet max Va is about 275V, but you can run them at 300-320V easily. Even if they wear more quickly, they are dirt cheap and many NOS good quality is available. I buy Svetlana Winged =C= EL90 for about 4€/pc
A 3rd option is that you use the Hammond 290CAX. This one has a 550VCT and a 630VCT secondary. With some planning and using a rotary switch or DPDT switch you can get 2 different B+ values. One for EL84 (550VCT) and one for 6V6 (630VCT)

[Paul-T]

FWIW you can perfectly use a 290-0-290V secondary for F*nder type amp builds....

with some planning and using a rotary switch or DPDT switch you can get 2 different B+ values. One for EL84 (550VCT) and one for 6V6 (630VCT)

I've been searching for a 290V secondary transformer for a while, for a Dual Lite. Unfortunately my vendor of choice has gone a bit quiet and paying over £100 would defeat the point of my budget build. But I've seen a promising 275 secondary. I'm not sure from the above post whether you're referring to 550V with SS rectification (I'm using EZ81) but do you think 275 would be fine? I've gone lower on a 5f2a and preferred the results but I can't see many references to slightly lower B+ on 18W builds. I'm planning power scaling in any case so losing a few watts doesn't bother me as long as the basic grunt is still there.

[JMPGuitars]

275-0-275 might be okay (there are some builds on this site using them), but you need to share the specs of the one you're looking at, and which version 18W you're building. The PT needs to supply enough current for all tubes.

[Bieworm]

I see you’re using the £ unit, so there’s abig chance you live in the UK. Well, Primarywindings is UK based. They have the PWAM01. I have used it on various 18W builds. But there’s no separate winding for a rectifier, and no center tap on the HT winding. So you’re bound to use a full wave bridge rectifier. The big plus on that PT is there’s the option to wire the HT to 0-275, 0-190 and 190-275V. A switchable B+! Works great!
IMHO you don’t need that EZ81 on an 18W
And don’t worry, the 0-275 yields plenty of voltage for the B+, more so with a diode rectifier

[Paul-T]

275-0-275 might be okay (there are some builds on this site using them), but you need to share the specs of the one you're looking at, and which version 18W you're building. The PT needs to supply enough current for all tubes.

I have used a few Primary Windings and indeed gave one of those transformers away. I really, really fancy tube rectification as a contrast to the Park 20W which this amp is replacing. Although I guess I could use two diodes and the EZ81 maybe, albeit that's a bit more complicated.

The PT I'm looking at is a vintage Radio Spares, 100mA secondary, two 6.3V taps. It would be about the same price as as PW.

My guess is that a 5 per cent difference would have a marginal effect but I'm ready to be enlightened as to how wrong i am.

[Paul-T]

Sluckey reckons this is right in the Goldilocks zone, so I'm picking up tomorrow.

[Bieworm]

The fancying is in your head. I bet you wouldn’t hear the difference between an EZ81 and a diode rectifier in a blind test, all other things equal (voltages etc)
EZ81’s don’t sag.

[JMPGuitars]

EZ81’s don’t sag.

They do, but it's subtle compared to other rectifier tubes. A sag resistor can certainly make up the sag if it's desired.