Debunk (or not) my little theory - rect tube socket

[pjd3]

Yup and I do.

I'm surprised that I didn't think in terms of the B+ being there on both sides of the resistor. But, it makes sense now, and I did in fact just check to see that it is there with my own eyes and meter.

I feel a little dumb but, as it turned out, I did have a good Belton solder lug socket for the rectifier tube. I did inspect if thoroughly and move any adjacent wires and even bend out tabs that looked a little close. The "bad" sockets are there waiting for the EL34's but, those sockets are coming out and in the barrel. I have new ones coming from AP. I did find a secondary wire with a piece of a crimp pin showing at the top of the heatshrink. I don't think it was that but, not taking any chances I made sure it and all else was thoroughly covered with insulative wrap. The amp did turn on fine with the GZ34 installed and remained that way for a while, holding B+ at a consistent 490vdc.

Thanks everyone, thats what this big oak board was for, to hit on deeper levels of setting up amps, as opposed to following classic layouts, circuits and modification.. My hope is to learn a bit more, and to not loose expensive transformers as I "develop character".

You have all been extremely helpful along the way.
Thank you.
Phil D.

[pdf64]

Ok, sorry I misunderstood.
So you have a .5 amp in each leg of the HT, right?

If so that’s 1 amp total …

707mA total.

[Stevem]

So your total D.C. Current draw with two output tubes is .707 amp?

[pjd3]

I could be way off but was under the impression that for EL34,

Plate current max = 140ma

Screen current max = 50 ma

My PT HT current rating is 200ma.

I can't recall why I used .5 amp fuses for the legs of the HT coil, either I saw the value used for some other amps or I simply or erroneously approximately doubled the expected max current that may be seen in either of the HT sides under some terrible circumstance.

The way I have been shown to derive PT primary fuse value is to crunch out the V/A of each secondary coil, add them up and double it. I may have done approximately the same for the HT coil/sides.

Phil D

[pjd3]

I missed something there, think it was to divide the sum of the secondary coil V/A's by 120v. (I = P/V) Then double that for a reasonable fuse current value.

Phil

[Stevem]

Here’s some basic info.
Say you want to make 60 watts RMS out of two output tubes, like 6550 output tubes.

Now we are just taking the output section here, do keep that in mind.

Power supply wise you would need .240 amp available to feed the center tap ( both output tubes ) of the OT, and 500 volts to get that job done.

Remember ohms law here!

If your chosen output tubes could handle a good amount more plate and screen voltage then you could make that same 60 watts by means of more voltage and less current.

This is all assuming a OT with a primary of 4.3 K ohms.

You don’t need anywhere near .700 amp of dc current to power the whole amp, preamp tubes included.

[pjd3]

Thanks Stevem, thanks for outlining that out,

However, I'm a bit confused where the .707 amps came from. My power supply (at least for this setup) is rated at 200mA and I expect that to never go higher than that, and likely never even near it, since this project ultimately is about a reasonably clean tone that wouldn't be pushing near its limits. Whether I've decided on EL34's or KT77's, I figured any tube at any time wouldn't be seeing a whole lot above 120ma or so. That for me is just an erroneous assumption based on seeing EL34 data that around 170ma is its max.

I'll fill you in a little more. Ultimately, this amp is to be a stereo amp with FX send/return that is specifically for a baritone guitar I'm about half way through building that is for electric fingerstyle, only. I found it very enticing to be able to play all the celtic and modern fingerstyle material that I learned to play during the covid era and make it work favorably with an electric guitar. That's why I choose pickups for the guitar with very low DCR and inductance, in an attempt to tilt the tone or EQ more towards a higher resonant frequency. And in my search for a favorite tube clean channel, the Bogner Shiva kept consistently calling me back. I thought is sounded beautiful with a Fender type guitar and some good quality stereo chorus/delay/reverb. The plan is to have that all coming out of the stereo guitar amp. I've already picked out what I believe could be the PT, an Edcor with 400mA HT current, 70v bias tap, and two separate 6.3vac @ 4 amps (the filaments for each side). I'm losing my mind, but having a raging good time doing it. And laying this amp out on a board was a way to force myself into learning more on an experiential level. I'm getting more than I bargained for already! But its kinda cool having to struggle, worry and ruminate over each element of the amp as it works its way to, something. When I arrive at a sound that impresses me, and can live with, I'll move it into something permanent. Its a long project for sure, but that's what I like about it. Ain't no simply quickee, at least not for me.

Perhaps I'm not clear on the HT fuse value I put in but, I figured a 200mA HT coil could pop a 500mA fuse if something arcane was to happen. I assume that .5 amps is .5 amps even if there are 2 fuses, one on each leg. However, if there was an "incident" on one side of the HT center tap, I'm not 100% sure how that would translate to the other side of the CT. Wouldn't the overcurrent event just be on one side? Or would it try to balance and bring that over current through the whole thing? Something to keep me awake at night.

Thanks again all for giving up some time and effort to get me thinking and considering things. I pray, that I can get through this without having to explain to my wonderful wife why "I just need another power transformer". Ha, I can think of more fruitful battles to win!

Thanks all
Best,
Phil D.

[pjd3]

For poops and giggles,

I did a search for "Transformer primary fuse popping with unloaded secondary. Below is the link if interested.

https://electronics.stackexchange.com/q ... at-no-load

Whether or not this is what was at the source of my opening primary fuse is, even with a fairly unloaded secondaries, I found this explanation interesting and worth considering. And it echos sentiments from an associate technician who is much more experienced than I, that I should take a look at where the AC input wave is when the switch is hit. I did set my scope up to look at this but, for reasons I'm not aware of, the fuse didn't blow over 20 flips of the AC switch.

But, the article shows a simulation transformer using a schematic drawing, and how the core can saturate more fully or quickly unloaded that loaded. I almost hope this was the case as it would remove my enormous mystification as to "what I have done wrong" status.

I may hold off and set up the power nodes, apply them to preamp and power tubes, and see if anything changes.

Phil D.

[pdf64]

... I assume that .5 amps is .5 amps even if there are 2 fuses, one on each leg…

With that arrangement, as I recall, the max combined HT current will be 1.414 x 0.5 = 0.707A.

The max HT current is determined by the HT voltage and OT impedance. The valves will loyally conduct far more current than is good for their long term survival.

50W amps use 2A time delay anti surge IEC spec primary fuses on a 240 mains supply, so it seems to me that on a 120V mains, a higher value would be required. Plus the change from IEC spec fuses to UL spec will require the value increasing. And definitely with a time delayed fusing action.

A light bulb limiter would be extremely beneficial for what you’re doing.

[jabguit]

Thanks Stevem, thanks for outlining that out,

However, I'm a bit confused where the .707 amps came from.

Half of square root of 2.

cheers,

[pjd3]

So, I went back and calculated the current that is expected in the primary winding based on the added V/A of each secondary coil divided by 120 acv.

Its an Edcor XPWR096

670 @ 200ma
6.3 @ 4 amps
6.3 @ 4 amps
6.3 @ 2 amps w/ct

The total V/A divided by 120 = 1.64 amps, if I did that right.

So perhaps its looking like a 2 amp fast blow fuse may not be large enough. Its my understanding that conventional wisdom at least doubles the calculated rating of the primary current which would suggest a fuse value closer to 2.5 amps, or maybe even 3 amps would be safe. And, if a slower reacting fuse is the proper type for this, then I shouldn't be surprised that my 2A fast blows are blowing!

The post from pdf64 tells me that I ought to do a little more research on how the fuse types and standards are evaluated and applied.

Thank you all once again for a generous outpouring of important and relevant knowledge. Its really something to be able to get such good input on the viscitudes of amp building incidents!

I"m still reading over all of these posts to make sure I"m getting it all!

Thank you all,
Best

Phil Dl