Calcultaing output power for Supro T-bolt

[tubeswell]

So I am checking whether the 15" https://taweber.powweb.com/weber/15f150.htm 30W speaker I got the other day will be able to handle the power output of a '65 t-bolt circuit I am planning on building next (or whether I need to order the 50W version).

Here are my assumptions (using the voltages courtesy of Weatherford Amps schematic attached herewith)

Pout = [(3.2 x Eo)^2]/(8 x Rpp) (is this the correct starting equation?)

Where: Eo is the (assumed) plate voltage swing (assumed to be about 370 with a plate idling at 410 - is this unreasonably high?), Rpp is the plate-to-plate load resistance, 2 x cathode-biased 6L6s biased to 75mA (tube current) each, Pv-Kv = 380, OT turns ratio 625:1, speaker = 8R, B+ = 410

So:

Pout = [(32. x 370)^2]/(8 x 5,000) = 35W

Or would anyone change these assumptions/equations?

Any help/criticism much appreciated. TIA

[es345]

I don't now your formula, this would be my way of estimation:

sinus input assumed
Ub=410V
minimum peak voltage between plate and cathode before starting clipping ~50V
maximum cathode peak voltage to ground at that point, 250 mA assumed ~50V

that results at in 310 V peak at the active half of the primary >> Pout = (Up*Up)/2*(Raa/4)=(310V*310V)/2500Ohm=38,4 W

So the results are in the same ballpark.

I would not use that 30W speaker; when you are going into overdrive, you can kill that one.

One add. comment: the OT turns ratio is 25, the impedance ratio is 625:1

[tubeswell]

One add. comment: the OT turns ratio is 25, the impedance ratio is 625:1

Right you are - it was a long day yesterday. I should've said imp ratio, but my brain got muddled up with the exponent in the other formula

Bugger! I'll have to order a 50W speaker

[greekie]

Can your OT even deliver that much power? The old ones sure couldn't!

If you are building it, hold the order for the speaker, and try a dummy load at first, to check it. Might save you some cash!

YM2C

[tubeswell]

The OT I have for this build is custom-made (but is bigger than the standard OT methinks). However I will hold off on the speaker order until I have run it on a dummy load as you suggest.

[J M Fahey]

Loved the schematic, gess will be a killer amp.
Math is good, maybe real world parts will provide somewhat less, say 30/32 "clean" watts, anyway more than enough to kill a 30W speaker when overdriven.
Considering you live almost as far as physically possible from Weber's shop, I'd play it very safe.
Good luck with your project.

[Zer09]

Tubeswell,
I'm actually getting ready to build this amp. The schematic I found was the Weatherford one you have. After taking a quick look at it I saw the the voltage list and 3.3v on the 6l6 plates. Did you come across any other errors in the schematic? Also please let me know how you like the Weber 15".

Thanks,
Zer09

[soundmasterg]

I'll have to email Sean to fix the error on the voltage chart there for the 6L6 plates. Somehow none of us noticed that before. The info I gave him for the schematic was based on restoring three vintage examples, all of which sound fantastic. If I was going to build one of these amps, I'd do it in a head...then I could pair it with different cabinet choices easily. They sound fantastic with 4x10's.

Greg

[Zer09]

Greg,
Thanks for the response. I figured that was the only issue because I've found it on several forums with no complaints.
As for cab style I was going to do a head because I prefer the portability. I just don't have a 15" cab to use with it and I have a 2x12 where my band practices. If it sounds better with a 4x10 I have one of those as well.

All of the 6l6 amps I build anymore, I build with the capability of using both 6l6's and el34's just because every 6l6 amp I've built at some point I've wanted to try el34's. My 5e8a Twin sounds amazing with both 5881's and el34's. The problem is that I've only done this fixed bias. Any experience with either 5881's or el34's in this circuit? My plan was to have switchable parallel resistors with the default being a 5881 set up switchable to el34. That way if the switch fails I'd be stuck with the 5881 setup and at the very worst I'd have cold biased el34's.

Rich

[soundmasterg]

Greg,
Thanks for the response. I figured that was the only issue because I've found it on several forums with no complaints.
As for cab style I was going to do a head because I prefer the portability. I just don't have a 15" cab to use with it and I have a 2x12 where my band practices. If it sounds better with a 4x10 I have one of those as well.

All of the 6l6 amps I build anymore, I build with the capability of using both 6l6's and el34's just because every 6l6 amp I've built at some point I've wanted to try el34's. My 5e8a Twin sounds amazing with both 5881's and el34's. The problem is that I've only done this fixed bias. Any experience with either 5881's or el34's in this circuit? My plan was to have switchable parallel resistors with the default being a 5881 set up switchable to el34. That way if the switch fails I'd be stuck with the 5881 setup and at the very worst I'd have cold biased el34's.

Rich

I think that is the only issue with that schematic. Sean pretty much got it nailed with input from me and a couple others with the exception of that boo-boo.

I wouldn't say it sounds 'better' with a 4x10....it sounds good no matter what you plug it into, but that happens to be one of the better sounds in my experience with those amps.

I've never tried EL34's in these amps...I'm sure they would work fine, given the proper cathode bias resistor, though the OT impedance may be a bit higher than what an EL34 pair would like to see. I didn't measure the OT Zed on the three amps I worked on, but I have an OT here that I can check...it may take me awhile though as school has me buried at the moment. I will say that Sovtek 5881's love these Tbolts and sound really good in them. It may be because the stock setup is biased pretty hot and that is how those particular Sovtek tubes like to be biased....hot.

It lists the proper transformers on the schematic so with those plus the circuit info, you should be able to build a nice sounding amp.

Greg

[tubeswell]

I didn't measure the OT Zed on the three amps I worked on, but I have an OT here that I can check...

A while back loudthud measured a T3778 OT he had (in response to an earlier query I had) = 5k

http://music-electronics-forum.com/t21526/

[Zer09]

I came across that post by Loudthud and that's what I was going from. I'm going to split the difference and use a 4.2k that I have laying around. i considered using a different value combined with an impedance selector switch but this actually puts me closer. We'll see how it sounds.

[soundmasterg]

A 4.2k should be just fine with 6L6 or EL34, though both of those tube types would be happier with something a little higher when cathode biased. It will work fine though and you might even get a little more power out.....

Greg