Fundamental question: Tubes, OTs and wattage.

[flood]

so I ordered 5 sets of the following transformers:

Power:
275-0-275 150mA
27V 50mA
3.15-0-3.15 5A (!)

Output:
8k CT 25W 0-4-8-16 sec

i was fantasising what to do with them after i'm done with my soldano build in december.

at some point of time i wondered if i could actually use other tubes - not only EL84 and 6V6 using the same output transformer. i googled around a bit and was stunned to find this:

http://www.hammondmfg.com/1608.htm

the transformer in question is the 1650F with 7.6k CT and 25W; it claims to handle 6L6GC and even EL34 in push pull. how is this possible? to my n00bish knowledge, a push-pull pair of these tubes would be pushing a minimum of 30W-50W. does it depend on voltages and bias?

i would LOVE to build a few different amps using different power tubes, particularly 6L6 and EL34. another idea i've had for a while was to use 4x 6AQ5.

what is possible and what is not? thanks for reading this and taking the time to address a question as superfluous as this...!

[Bob-I]

Yes, output power depends on voltage, bias, phase invertor, xformer etc.

At the voltage you'll be running, you might be pushing the limit of that OT but I wouldn't be too worried about it. The xformer will saturate and most likely give you a nice compression, but the likelyhood that you'll do any damage is small.

Make sure to adjust for the impedance of the tubes. Check the tube charts for AB impedance and use the correct setting on the OT.

Good luck on the project.

[tubeswell]

Yep most transformers can take a bit of 'abuse'. A power-handling rating is merely more-or-less a reckoning of how much the thing will heat up before is self-destructs, and there's a lot of variables in that.

[flood]

thanks for the replies... stupidly enough, i didn't have the foresight to ask the winder to put multiple taps on the xformers. so all i have is 8k CT.

i suppose it would work for 2x6L6 with lowish plate voltages (300V or so)... i'm just not sure about EL34s, since i believe they like to see about half of that impedance.

i'm a newbie, so do be gentle if i have some basic facts mixed up somewhere.

[tubeswell]

thanks for the replies... stupidly enough, i didn't have the foresight to ask the winder to put multiple taps on the xformers. so all i have is 8k CT.

i suppose it would work for 2x6L6 with lowish plate voltages (300V or so)... i'm just not sure about EL34s, since i believe they like to see about half of that impedance.

i'm a newbie, so do be gentle if i have some basic facts mixed up somewhere.

You say 8k on the Primary, what is the OT secondary? You need to think of OTs as having a 'reflected load'. The load on the primary depends on the load at the secondary. E.g.; if the secondary was rated for '8R' with an '8K' primary, then hooking a 4R speaker up to it will give you a 4k reflected load on the primary and so on. 4k is good for a couple of 6L6s, or a quad of 6V6s. 2k is good for a quad of 6L6s, and so on. (Hooking up a 16R and an 8R in parallel will give you a 5k3 reflected load - although the 8R speaker will 'get' more power than the 16R speaker)

[paulster]

You say 8k on the Primary, what is the OT secondary? You need to think of OTs as having a 'reflected load'. The load on the primary depends on the load at the secondary. E.g.; if the secondary was rated for '8R' with an '8K' primary, then hooking a 4R speaker up to it will give you a 4k reflected load on the primary and so on. 4k is good for a couple of 6L6s, or a quad of 6V6s. 2k is good for a quad of 6L6s, and so on. (Hooking up a 16R and an 8R in parallel will give you a 5k3 reflected load - although the 8R speaker will 'get' more power than the 16R speaker)

Exactly. The Hammond single-ended wiring diagrams are good for illustrating how this works as they use a single primary and have 4 speaker outputs to take advantage of this.

[Phil_S]

thanks for the replies... stupidly enough, i didn't have the foresight to ask the winder to put multiple taps on the xformers. so all i have is 8k CT.

Though it's overkill, you can probably build a nice PP triode amp (12AU7. 6SN7) which will be very tolerant of it's transformer load. These want to see a load of something over 16K. Many say 22.5K is ideal. If you figure 2x as the multiplier, you have a 16K CT primary with 8, 16, and 32 ohm secondaries.


Or you can figure on a 4K primary with 2, 4, and 8 ohm secondaries...you might do with with a pair of cathode biased 6L6 or EL34, or a quad of 6V6 or EL84.

Use your imagination. Just remember the 25W capacity.

[flood]

You say 8k on the Primary, what is the OT secondary? You need to think of OTs as having a 'reflected load'. The load on the primary depends on the load at the secondary. E.g.; if the secondary was rated for '8R' with an '8K' primary, then hooking a 4R speaker up to it will give you a 4k reflected load on the primary and so on. 4k is good for a couple of 6L6s, or a quad of 6V6s. 2k is good for a quad of 6L6s, and so on. (Hooking up a 16R and an 8R in parallel will give you a 5k3 reflected load - although the 8R speaker will 'get' more power than the 16R speaker)

hi tubeswell,

thanks for the explanation... my OTs are custom wound. specsw are in the first post, but i'll just write them here: Pri 8k CT (4k-0-4k) Sec 0-4-8-16 ohms. so i can configure it according to pretty much any speaker combination.

the OT power handling is 25W.

[flood]

Though it's overkill, you can probably build a nice PP triode amp (12AU7. 6SN7) which will be very tolerant of it's transformer load. These want to see a load of something over 16K. Many say 22.5K is ideal. If you figure 2x as the multiplier, you have a 16K CT primary with 8, 16, and 32 ohm secondaries.


Or you can figure on a 4K primary with 2, 4, and 8 ohm secondaries...you might do with with a pair of cathode biased 6L6 or EL34, or a quad of 6V6 or EL84.

Use your imagination. Just remember the 25W capacity.

hey phil,

already did that the firefly was the first amp i buillt. i love it. i'm looking at an EL95 based blues express next (i opened a thread in trainwreck discussion, no content except for the schematic yet though).

using a 25W tranny on a 2W amp seems to be a bit... overkill though.

i really want to do an EL34 push-pull amp though. i could cold bias it a bit with 470 ohms on the cathodes (individual cathode resistors). anyway, i have quite some time - half a year, in all probability - till then, and a few amps on my plate... so i'll figure it out later. do keep the discussion going though!

[John_P_WI]

Just remember, even if you use large bottle tubes, they will not produce any more power than the power transformer can deliver. I think the biggest problem when people try to do this is that they run out of heater current first.

John

[flood]

Just remember, even if you use large bottle tubes, they will not produce any more power than the power transformer can deliver. I think the biggest problem when people try to do this is that they run out of heater current first.

John

noted... the PTs can supply 5A of 3.15-0-3.15V though. i'm looking for tonal variations, more than anything. once i'm done with 18-25 watters, i will move on to larger power...