Will AO-29 PT work for a PP 2xEL34 Plexi?

[High Voltage]

Long time lurker, first time poster so firstly let me thank everyone for all the amazing posts and information already shared on this site. I have already learned so much here!

I'm looking to start my first "from-scratch" build and really want to do a 50W EL43 Plexi (not 6V6). I have a Hammond H-AO-22111-4 PT from a AO-29 and was wondering if it will work?

Depending on where I read the PT B+ is rated at 150 or 170ma which by my research is just enough however I have read elsewhere in forums people saying that even for a 6L6 PP you need >200ma(?) However when I look at ClassicTone's plexi and 800 replacement PTs they are only rated at 150ma also so I figure the Hammond PT must be enough!?

Sorry in advance if this is a dumb question and i am missing something obvious but any guidance appreciated!

[sluckey]

I would not use a PT that was chosen for 6V6s in an EL34 circuit.

[High Voltage]

OK, thanks for the input. Can you point me in any direction to learn why it's not a good idea?

[shooter]

Can you point me in any direction to learn why it's not a good idea

The easiest is "original design", which in your case was 6V6
the next place is the tube datasheets, comaping things like operational conditions of the "original design" an your new design, NOTE the difference's
the next would be "other similar" designs

this a good start for PS design;  But there is ALOT of moving pieces beyond that.
http://www.valvewizard.co.uk/psu.html

[High Voltage]

Thanks for the info. I can't find anything that jumps out saying it wont work, but i'll keep looking.

- steve

[sluckey]

Check the impedance ratio of the OT. I suspect it is not ideal for EL34s.

Search Google for "calculate output transformer impedance" if you don't already know how to do this. I like to use a variac and a 6.3VAC transformer. Set the variac to provide 1VAC to the OT secondary. Measure the voltage across the plate leads of the OT. This is the voltage ratio. Now square this number to get the impedance ratio. Now multiply this impedance ratio times the speaker impedance. This will be the primary impedance that the tubes will see.

[66Strat]

EL34 tube current requirements for AB1 operating points as those found in a 1987 50 Watt Marshall circuit are much greater than 6V6 operating points. For fixed bias AB1 operation the power transformer will need to supply at least 260 milliamps of secondary winding current capacity. El34s like a reflected load of about half that of 6V6 tubes with a much greater current capacity. You might get away with cathode biased 6l6G or 5881 tubes with the transformers that you have for a JTM45 type amp. However IMO, your best option would be to build a 6V6 version of the Marshall circuit.

[shooter]

You might get away with cathode biased 6l6G

or not, use the data sheet pointed to as an example above.  the Ia is ~~ twice the fixed bias specs.

you can cheat the OT with a 4ohm speaker on the 8ohm tap, but the dynamic "power"(heat, current, voltage, etc) still needs "compensated" for

[66Strat]

You might get away with cathode biased 6l6G

or not, use the data sheet pointed to as an example above.  the Ia is ~~ twice the fixed bias specs.

you can cheat the OT with a 4ohm speaker on the 8ohm tap, but the dynamic "power"(heat, current, voltage, etc) still needs "compensated" for

If the load and screen voltage are managed within 6L6G specs, 150 ma. will work.

[High Voltage]

EL34 tube current requirements for AB1 operating points as those found in a 1987 50 Watt Marshall circuit are much greater than 6V6 operating points. For fixed bias AB1 operation the power transformer will need to supply at least 260 milliamps of secondary winding current capacity. El34s like a reflected load of about half that of 6V6 tubes with a much greater current capacity. You might get away with cathode biased 6l6G or 5881 tubes with the transformers that you have for a JTM45 type amp. However IMO, your best option would be to build a 6V6 version of the Marshall circuit.

Thanks for this and for the other replies. Just wondering how you get to the 260mA value? Admittedly im still learning but when I do the calculations for AB1 I get around 120-150mA depending on the setup. The datasheet ive been using most is this one from Philips: https://frank.pocnet.net/sheets/030/e/EL34.pdf

Also, im confused because if I search for drop-in replacement transformers for Plexis/800s they all seem to have a B+ around 150-200mA's (eg http://www.classictone.net/Marshall-Style-Transformers.html ). In fact i've never actually seen one over 200mA let alone anywhere near 260mA. Im sure its just a gap in my understanding but I would super appreciate if someone could let me know where im going wrong??

[shooter]

around 150-200mA's

look at the datasheet, fixed bias (2 X 110mA)

my understanding of Marshall, you want a tight, save the sag for Fender, PS
so 200mA of good iron would be a good comprise

[High Voltage]

It appears i've been calculating for cathode bias. My bad sorry. Thanks for your help.

I still am really curious how the vast majority of clone PTs are 150mA's (only a few are 200mA's). Not least because I'll need to buy one if I don't use the AO-29 PT, and it would be a bit dumb to buy a 150mA PT if I already have one. Sorry if i sound a bit neurotic but these things burn on me when I cant figure something out :)

[shooter]

calculating for cathode bias

Nothing wrong with Self biased, I build 99% using self biased.  My take on the plexi was with PP self biased EL34's.  I'll try n find the data - if I wrote it down 
I happened to have Marshall 100W iron on hand so I went that route, the iron just yawned at full roar 
What is your "goal" using 34s over 6V6?
I'd argue there won't be much/any "volume" bump.  I'd also argue the 6V6 tone will be better at the "matched" current available, but if I only had 34s and peanut butter to eat, try it self biased cool-ish and monitor temperature over time

[High Voltage]

Actually I recently bought the new/recent Marshall SV20 which is cathode biased EL34s with low plate voltages (in the 220V range) to put out around 20W. It is one million percent classic Plexi! Perhaps the best amp i've ever owned or used. Better than my all-original 2203 even. So I totally agree cathode biased can sound great. If you do find your data on the cathode PP build that would be great!

My end-end goal is probably something closer to the SV20 than the plexi (1987), but I really want to create the classic plexi first as a learning exercise and and a reference point for future builds.

Im still confused why all the most of the plexi/800 clone PTs are 150mA! :)

[shooter]

I was just beginning to document my builds properly about this build so I know I made changes, but they were tonal tweaks so the DCv numbers are probably close.

when I cover a classic I try and "capture" the essence, in this example, V1 and the ability to mix/blend.  After that my abstract nature takes over 
so, the PI is from Gibson, the PA from Mesa.

[High Voltage]

That looks super cool! How did it sound?

What is the send/return around the TMB? Is that for a tremelo or reverb? or an effects loop?

[66Strat]

Here's a link to an analysis of the 50 Watt JCM 800 circuit.
https://www.ampbooks.com/mobile/classic-circuits/marshall-jcm800-2204-power-amp/

The 50 Watt Marshall 1987, 2204, and JCM 800 circuits are my favorites. I really like the idea of a cathode biased version of one of these.

Here's the Phillips data sheet section containing the Class AB1 operating points. I circled the current demand at full signal for 58 watts fixed bias operation. I also circled the full signal current demand at 35 watts for cathode bias operation. These figures are comparable to the Mullard datasheet that I posted earlier.

I also have a link to the Hammond replacement transformer for the JCM 800 and JMP 50 watt amps. It's interesting to note that the Hammond transformer is rated to provide 690 volts to center tap at 150 milliamps, and a maximum current rating oh 213 milliamps.
https://www.hammfg.com/files/parts/pdf/290GX.pdf

[shooter]

It ticked off all my expectations, I don't play so I left it with my Son, he had his group beat on it while I avoided snow, after a couple minor tweaks it sold to a very good guitar player; his comment went something like "I've been chasing that tone for 17yrs".  I broke even 

the FX was a troubleshooting "feature".  didn't make it in the final cut.  that was the final version of a 2 year, 3 build experiment.
note: like everything in art, it's not for everyone, the PI "adds" some interesting mojo that requires a player to adapt play instead of forcing the amp but that's the cool thing about a good player, that's not a thing.

choke your 34's to around 350vdc plate, dial the dissipation to ~~ <24W a tube and you can get by with your 150mA, the problem is filament current in my mind so find out if your PT was undersized in the original design and don't expect it as a daily bread winner

[66Strat]

I was just beginning to document my builds properly about this build so I know I made changes, but they were tonal tweaks so the DCv numbers are probably close.

when I cover a classic I try and "capture" the essence, in this example, V1 and the ability to mix/blend.  After that my abstract nature takes over 
so, the PI is from Gibson, the PA from Mesa.

Nice! I love Frankenamps.

[High Voltage]

he Hammond replacement transformer for the JCM 800 and JMP 50 watt amps. It's interesting to note that the Hammond transformer is rated to provide 690 volts to center tap at 150 milliamps, and a maximum current rating oh 213 milliamps.

Thanks for this info its super helpful! So I think this might potentially be the answer to my confusion around the clone PTs being 150mA.

So my question is what *exactly* do the 150 & 213mA numbers mean... It would totally make sense given the Ia of the EL34s in PP fixed bias if its peaking out and sagging at 213mA, but then that begs the question what is the 150mA number? And reading the spec sheet it sounds to me the 213mA is the safe max current, not to be exceeded?

[sluckey]

There are two different AO-29s and they have PTs with quite different ratings. Which one do you have?

[66Strat]

Thanks for this info its super helpful! So I think this might potentially be the answer to my confusion around the clone PTs being 150mA.

So my question is what *exactly* do the 150 & 213mA numbers mean... It would totally make sense given the Ia of the EL34s in PP fixed bias if its peaking out and sagging at 213mA, but then that begs the question what is the 150mA number? And reading the spec sheet it sounds to me the 213mA is the safe max current, not to be exceeded?

You're very welcome.

The 150 ma. number is regarding the secondary AC voltage supply. With 120 volts AC (line voltage) applied to the primary, the power transformer will supply 690 volts AC to center tap (345-0-345) under a 150 ma. load. This is not the max current rating of the transformer. The 213 ma. number represents the maximum current limit for the secondary winding of the transformer. Anything under 213 ma. should be safe. However, the transformer will supply something less than 690 volts at that current. My best guesstimate is that the transformer in the Marshall circuit will sag sufficiently to prevent secondary current from exceeding the limit. At least that's how I interpret the data.

[High Voltage]

The 150 ma. number is regarding the secondary AC voltage supply. With 120 volts AC (line voltage) applied to the primary, the power transformer will supply 690 volts AC to center tap (345-0-345) under a 150 ma. load. This is not the max current rating of the transformer. The 213 ma. number represents the maximum current limit for the secondary winding of the transformer. Anything under 213 ma. should be safe. However, the transformer will supply something less than 690 volts at that current. My best guesstimate is that the transformer in the Marshall circuit will sag sufficiently to prevent secondary current from exceeding the limit. At least that's how I interpret the data.

Thanks for the brilliant explanation. I totally get it now!

[High Voltage]

There are two different AO-29s and they have PTs with quite different ratings. Which one do you have?

I have the bigger one. Its enormous! If i ever have to throw a body in a lake im gonna tie it down with this PT.

[jbefumo]

Old topic, but new to me... I thought we were discussing power transformers, not output transformers.  I have two power transformers, and the question I'm trying to get answered is whether they were overbuilt, as so many things were in those days, to deliver more current than was being used by the two 6V6s, plus the various preamp tubes (or, same question regarding the reverb transformer and its EL84s)? I'm building a Stout TMB, and am trying not to buy any parts. I have a pair of KT66s and an appropriate OT, and might like to use it with one of those Hammond transformers. Alternatively, my fallback plan is to build it with a pair of 6973s I have on hand and an EL84 OT.

Check the impedance ratio of the OT. I suspect it is not ideal for EL34s.

Search Google for "calculate output transformer impedance" if you don't already know how to do this. I like to use a variac and a 6.3VAC transformer. Set the variac to provide 1VAC to the OT secondary. Measure the voltage across the plate leads of the OT. This is the voltage ratio. Now square this number to get the impedance ratio. Now multiply this impedance ratio times the speaker impedance. This will be the primary impedance that the tubes will see.