100ma + 350v secondary's

[RockinRocket]

I have a power transformer that is rated at 100Ma + 350v secondary's that makes about 450 dc with a Gz34. I used it in a JTM 45 and it sounded good but sagged to much for authentic JTM45 tones.

Any idea what power tube would be suited for this? Looking to build a 3 12ax7 preamp.
Would a 12bh7 be suite for the power? Also I might use a pair of kt66 or el34s but would I risk damaging the power transformer running max vol, set to 10?

[brewdude]

6V6?

[roberto]

What OT do you have?

Let me explain you my question: I'd rectify in order to have 250Vdc, not to exceed 100mA with a 8k Raa (EL84, 6V6), maybe lowering a bit the screen voltage.
Going to 450V you would need a 16k Raa not to exceed 100mA, but you'll need a custom OPT, a more expensive PSU, unusual tubes, etc...

EDIT: further explanation of the question

[Stevem]

Something's not right with your numbers'!

The conversion factor of PT A/C voltage to D.C. By a 5AR4 is 1.3.
This equates to 390 volts, not 450 by a long shot!

Here's a good rule of Thumb to work with in terms of power output stages for a fixed bias amp.

If for example your chosen output tubes need 100 ma to produce 18 watts, then once you add in the current needed by the preamp tubes you should factor in a added 25% more current if you want the amp to still have very good punch and dynamic's when it's very cranked up.

In terms of 6V6 tubes biased on the hot side you would need like 90 ma just for that pair to have some 18 watts of clean power.

For each preamp tube used and powering both sides you need a additional.003 amp .

Also keep in mind that it's voltage and current that make for power, so if your working with a PT that's a little shy of current for your needs , but has 45 more volts or so then what you need, then that's not all too bad, not ideal, but workable.

Just note that it will run a tad hot .

Also note that a 12BH7 is a 9 pin preamp tube, not a output tube.

Also as requested we need the details of your OT to help work out what output tubes you can use, but as I posted you do not have the needed current for output tubes that can do more then 15 watts each max!

You should buy yourself a copy of the RC-30 RCA tube manual, as it will help you greatly!

[roberto]

Something's not right with your numbers'!
The conversion factor of PT A/C voltage to D.C. By a 5AR4 is 1.3.
This equates to 390 volts, not 450 by a long shot!

350 x 1.3 = 455.

Here's a good rule of Thumb to work with in terms of power output stages for a fixed bias amp.
If for example your chosen output tubes need 100 ma to produce 18 watts, then once you add in the current needed by the preamp tubes you should factor in a added 25% more current if you want the amp to still have very good punch and dynamic's when it's very cranked up.

It really depends on too many factors to just attribute to it the "rule of thumb" nomea. Rectification, response, capacity of each node, etc...

In terms of 6V6 tubes biased on the hot side you would need like 90 ma just for that pair to have some 18 watts of clean power.

Being in class AB for the full power, the maximum current is independent on how you bias the tubes.

For each preamp tube used and powering both sides you need a additional.003 amp .

1.5 mA per triode is a bit over the average. You can get it for CF at high voltages, in some cases the PI, but other stages are way lower.

Also keep in mind that it's voltage and current that make for power, so if your working with a PT that's a little shy of current for your needs , but has 45 more volts or so then what you need, then that's not all too bad, not ideal, but workable. Just note that it will run a tad hot .

If you run it properly it won't run hot, plus 45V can become out of specs especially for modern screens. It cannot be generalized especially when we are talking about EL84 or 6V6.

Also note that a 12BH7 is a 9 pin preamp tube, not a output tube.

There are no preamp tubes or output tubes, just ways to use them. Check the datasheet, page 2: https://frank.pocnet.net/sheets/127/1/12BH7A.pdf

[Stevem]

Even though a 12BH7 could be used as a Low wattage output tube, being that it’s a Triode it will respond and sound very different then any Tetrode or pentode type output tube, especially when driven hard!

[Phil_S]

Look into using a 5Y3GT with a pair of 6V6. I think that will work.

[roberto]

Even though a 12BH7 could be used as a Low wattage output tube, being that it’s a Triode it will respond and sound very different then any Tetrode or pentode type output tube, especially when driven hard!

This is totally different than saying it's a preamp and not a power amp tube. Every tube sounds differently to others even considering only pentodes or tetrodes.

[Stevem]

Let me try to please you yet once again Roberto!
I was mainly referring to the huge difference in the load line curves between Triodes and the other tubes.

[roberto]

Let me try to please you yet once again Roberto!

It's not a matter of pleasing anyone Stevem, don't feel offended by my words, it's just a matter of reporting the right things with the right words.

I was mainly referring to the huge difference in the load line curves between Triodes and the other tubes.

Loadline is the same with triodes, pentodes, eptodes...

[didit]

Full wave choke input silicon rectified should provide around 340VDC @ 120mA, which would run up a pair of 6V6 right. Hammond 158Q costs less than the price of a socket plus a quality rectifier to stuff in it.

Read this for overview explanation and formula basics for your engineering arithmetic.

https://www.hammfg.com/electronics/tra ... /rectifier

Best .. Ian

[Phil_S]

Full wave choke input silicon rectified should provide around 340VDC @ 120mA, which would run up a pair of 6V6 right. Hammond 158Q costs less than the price of a socket plus a quality rectifier to stuff in it.

Read this for overview explanation and formula basics for your engineering arithmetic.

https://www.hammfg.com/electronics/tra ... /rectifier

Best .. Ian

With respect for this suggestion, it doesn't comport with my understanding. I could easily be wrong. I had thought about choke input. If the secondary output is 350VAC measured across both legs, then choke input will produce B+ ~160VDC and ~150mA. I didn't think that was "useful" at least in the mainstream of guitar amp builds. Did I get the math wrong? Choke input would be great if the PT is 350-0-350, not for 175-0-175. I think we are looking at the lower output. In any case, agreed the choke is inexpensive enough!

[didit]

Transformer that produces 450VDC with GZ34 must be, so yea.

Best .. Ian

[sluckey]

With respect for this suggestion, it doesn't comport with my understanding. I could easily be wrong. I had thought about choke input. If the secondary output is 350VAC measured across both legs, then choke input will produce B+ ~160VDC and ~150mA.

But that's not the case. This PT is 350-0-350. The first sentence in this thread says so in so many other words...

"I have a power transformer that is rated at 100Ma + 350v secondary's that makes about 450 dc with a Gz34."

[Phil_S]

Thank you for the correction! I missed the detail (devil that it is.) I agree then, choke input is a great way to tame this beast should do what didit (Ian) says. That should be adequate to keep up with a pair of bigger octal tubes than 6V6's. I did something similar a while back. There's a thread around here somewhere.

Ian: sorry, I should have got this. Thank you for being so gracious.