Output transformer rating

[TOA Bassman]

Hi, first post here so apologies if I'm in the wrong section.

Fifteen years back a bought a dead 1970 TOA HA-356 PA amp for £35 with a view to scrounging the transformers. To cut a long story short, I got it working (a resistor in the feedback circuit had gone super-high), decided it was all ok and forgot about it.

I recently dug it out, converted it to 5F6-A Bassman spec and it sounds great. I'm using Sovtek 5881 WGC valves (they're the only 6L6 type I can find that'll fit without modifying the chassis) with 455-ish Volts on the plates and about 32uA plate current. I know the Sovteks have a reputation for being near indestructible but I don't want to destroy the OT...

So, here's the question; given that the original TOA design is for continuous service and used a pair of EL34s at 450V plate Volts with the service diagram indicating 94uA through the OT at full chat with no input signal, how high do you think I could push the plate current without burning something out? I'm quite happy with the sound of the thing as is, just curious as to how high I could push it :-)

One other thing; the original TOA design had the input and driver ECC83s each running on its own separate 5V, 300mA ac heater tap. Any ideas why 5V? Again, I'm just curious - I've got enough current from the 6.3V tap to run a pair of 5881s ac and three ECC83s DC so I just taped the 5V taps up.

Thanks in advance.

[Chuck H]

Your scenario seems fine. Repurposing transformers like this is common. It's important that you monitor the 6.3V secondary and adjust calculations for modern mains voltage (if it differs from the schematic/original design). If the 6.3V should be, say, 7V using that amp with modern mains voltage, but you only have 6V, that 6 may seem close enough to spec for the tubes to ignore but it means the 6.3V secondary is being dragged down a full volt more than it's intended operation. Check this and monitor excess for heat in the transformer.

As to why 5V on the preamp tubes... Dunno. But it may be for reasons of tube life, which is well extended at low, but not grossly low filament voltage. That being the case it's possible the entire amp is designed for long service. In which case it may not ever be intended to produce maximum power. Much less overdrive and clipping for hours on end. You'll need to asses with your best guess whether or not the OT 'seems' large enough for this sort of duty. And, again, monitor for excess heat.

I'm not familiar with that amp, but provided that 6.3V secondary is indeed up to the work I don't think you have a problem.

[bob p]

I'm thinking that you meant to type "mA" instead of "uA".

If you're planning on running those 5881 in Class AB at Va=450, I'm thinking that 40mA / 70% would be about right. If you tried to push them up higher more you'd also be pushing the current rating on your OT, so proceed with caution.

Regarding those heaters -- I'm inclined to think that if someone wired up ECC83 on 5V taps then they chose the wrong taps. 5V is only 80% of the 6.3V spec, and anything less than 90% or over 110% will shorten lifespan.

edit: I just noticed that there are some differences in the calculated output between some of the on-line bias calculators, so be sure to double-check the numbers. My 40mA number would be per tube, 80mA for a pair, at idle.

[TOA Bassman]

Thanks for the replies gentlemen.

Bob P; you are correct of course, I meant mA. The 5V, 300mA taps are identified as such on the service diagram and, given that even using both would not provide enough current to run a GZ34, this was definitely a design feature. Do you mean 40mA per valve at idle or flat out with no input?

Chuck H; everything is running very cool and looking under stressed at 455V/32mA. The service diagram rates the 6.3V secondary at 4A, so even running all three ECC83 heaters DC (bridge rectifier) and a pilot light it shouldn't be struggling given the 'continuous service' nature of the design. I take the point that it was probably never intended to run much above idle as a PA amplifier but I'm hoping that the reduced dissipation of the 5881s compared to the original EL34s should give loads of spare capacity. I'd post the TOA schematic for you to look at but the organisation I got it from were keen not to have it released into the wild... You can request a pdf version here -
http://www.radiomuseum.org/r/toa_ha_356.html
if you're interested.

Thanks again.

[bob p]

I'd post the TOA schematic for you to look at but the organisation I got it from were keen not to have it released into the wild...

I've never understood why people feel as if they have proprietary rights to old obsolete schematics when they're not the copyright holder. If they don't own the copyright to the schematic then they are acting as if they have ownership of someone else's intellectual property when they don't.

I've encountered many unimportant people who try to make themselves important solely because they are hording something that actually belongs to someone else. If the schematics that they are holding should become widely available then those "important" document hoarders would be effectively marginalized and return to a state of being irrelevant; they act to impede the sharing of the data solely to preserve their artificially contrived level of importance, even though they have no ownership rights to the data.

In general we like to ask for schematics to be posted when help is being sought, to make it easier for those trying to help and to eliminate the need for guesswork. Suffice it to say that not being able to post a schematic for an oddball amp, when you have it, is going to negatively impact your ability to get support everywhere.

I would post it citing "Fair Use" and tell them to go pound sand.

[nosaj]

I've never understood why people feel as if they have proprietary rights to old obsolete schematics when they're not the copyright holder. If they don't own the copyright to the schematic then they are acting as if they have ownership of someone else's intellectual property when they don't.

I've encountered many unimportant people who try to make themselves important solely because they are hording something that actually belongs to someone else. If the schematics that they are holding should become widely available then those "important" document hoarders would be effectively marginalized and return to a state of being irrelevant; they act to impede the sharing of the data solely to preserve their artificially contrived level of importance, even though they have no ownership rights to the data.

In general we like to ask for schematics to be posted when help is being sought, to make it easier for those trying to help and to eliminate the need for guesswork. Suffice it to say that not being able to post a schematic for an oddball amp, when you have it, is going to negatively impact your ability to get support everywhere.

I would post it citing "Fair Use" and tell them to go pound sand.

Here it is.

schematic.pdf

nosaj

[TOA Bassman]

Here it is.

[ATTACH]44627[/ATTACH]

nosaj

Whilst I agree with the comments above regarding intellectual property rights, the Radio Museum site was the only place I could find a copy and they specifically requested that it should not be posted online. Since they didn't charge for the pdf I felt it would be churlish to ignore their request.

I agree that the more widely available this information is, the better it is for all of us.

Stupidly, I linked to the HA-355 anyway which is a completely different circuit. It should have been this one:

HA-356 Ampl/Mixer TOA Electric Co., LTD.; Kobe, build

[bob p]

Whilst I agree with the comments above regarding intellectual property rights, the Radio Museum site was the only place I could find a copy and they specifically requested that it should not be posted online. Since they didn't charge for the pdf I felt it would be churlish to ignore their request.

It looks like their site gets it's revenue through targeted advertising. The reason that they don't want the schematic to be available anywhere else is to ensure the future ad revenue for their site. By asking that the schematic not be posted anywhere else they're hoping to block competition for targeted advertising. The way they look at it, if they have a monopoly on the schematic then that helps to ensure their future ad revenue.

In the big scheme of things they're never likely to realize any ad revenue on that oddball schematic ... until you post a link to it in a thread like this one, and drive everyone from this site to their site. That schematic has not been an ad revenue generator until today's little surge in page hits. That little surge in page hits is why they ask you not to post the schematic elsewhere.

It looks like we have another case of a site trying to ensure their own ad revenue by offering restricted access to someone else's property. There are two sides to that coin. They have no right to do that, but it does help to fund a useful library.

[nosaj]

Whilst I agree with the comments above regarding intellectual property rights, the Radio Museum site was the only place I could find a copy and they specifically requested that it should not be posted online. Since they didn't charge for the pdf I felt it would be churlish to ignore their request.

I agree that the more widely available this information is, the better it is for all of us.

Stupidly, I linked to the HA-355 anyway which is a completely different circuit. It should have been this one:

HA-356 Ampl/Mixer TOA Electric Co., LTD.; Kobe, build

There was no requirement for me as a guest there. Which is why it's still watermarked.
nosaj

[pdf64]

... Do you mean 40mA per valve at idle or flat out with no input? ...

Unless the amp has a LOT of hiss etc, the 2 conditions are identical; regular user vol/tone controls generally don't affect the operating point of things.

[TOA Bassman]

Thanks for the replies. I didn't mean this to turn into a discussion about intellectual property rights :-) Item 3. of Radio Museum's Membership conditions states "You will not pass on to a third party any data, pictures, etc. which did not originate from you. You are loyal to Radiomuseum.org." Whilst I don't think they have any rights to the information I don't think they have any obligation to share it either so, out of courtesy, I linked to their site rather than post the schematic.

It seemed to me that if the original circuit called for 450V/47mA per EL34 (which is very high by Dan Torres' charts!) there's no reason why the OT shouldn't handle 450V/47mA per 5881. I don't see any point in pushing the valves that hard, I just wondered if there was anything I hadn't considered.

[bob p]

Sorry if I hijacked your thread with the intellectual property concerns. That BS just rubs me the wrong way.

I think you've done a good job thinking things through. I doubt that you're going to run into any surprises.

[TOA Bassman]

Sorry if I hijacked your thread with the intellectual property concerns.

No need to apologise, thanks for your advice.

[pdf64]

...It seemed to me that if the original circuit called for 450V/47mA per EL34 (which is very high by Dan Torres' charts!) there's no reason why the OT shouldn't handle 450V/47mA per 5881. I don't see any point in pushing the valves that hard, I just wondered if there was anything I hadn't considered.

Yes it does seem unnecessarily high idle dissipation, especially given the application.
I guess those old 6CA7 were tough and could take it.
And we should be grateful that the manufacturer bothered to provide the info, instead of leaving it to guesswork / assumption.
Modern EL34 (and 5881) would probably be better off biased a bit cooler.
I suggest to try and measure the OT primary impedance.

If considering 5881 in place of 6CA7, the max available Vac output from the cathodyne may be insufficient to fully drive them; to fix that its HT dropper R44 might be reduced in value.
Also the g1-k resistance is over the 5881 limit of 100k, so R33 and R34 should be reduced appropriately, and the bias voltage will be too low, so the R40 and R41 values may need tweaking.

Whilst tinkering, the addition of HT etc fusing would be a good thing.

[bob p]

i thought he said that he had gutted it and rebuilt it as a tweed bassman. if that's the case then he's already rebuilt the PI to provide adequate drive for 6L6/5881...