Amp builders: Need some assistance on a project

I have the following Heyboer PT that is in need of a proejct:

Heyboer Hy022772 PT
HV is 370-0-370 @ ~.060mA (probably more like .075mA)
6.3v @ 1.5 Amp and 5v @ 2.5 Amp

Originally, I had this in a scratch build 5f1 but the plate voltage was way high (400+). I couldn't bring it down with a bias resistor, so I ended up swapping it out for something closer to spec. Everywhere i look it says this PT is used for champs and princetons, but I just couldn't make it work in my build.

I though maybe a 5e3 circuit, but from what I've read, the voltage is in the ballpark, but not sure about the current. Any thoughts?

martinman said:

I have the following Heyboer PT that is in need of a proejct:

I though maybe a 5e3 circuit, but from what I've read, the voltage is in the ballpark, but not sure about the current. Any thoughts?

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That seems like pretty low current rating for the high voltage tap. Assuming 14w max dissipation for 2 6V6s, you'd have ~76ma draw for the power tubes and ~4ma dissipation for 2 12AX7s. If we assume that higher end you quoted on the Heyboer and take 76ma as the rating for the transformer, you'd be pushing it a bit.

With that said, that assumes max dissipation. Typically you'd run them a little less, maybe around 70%. So:

(14w * 0.70) / 350v = 0.028A * 2 = 0.056A = 56ma + 4 ma = 60ma

So for the power tube plates and screens, plus the preamp tube plates, you should be good with the tube compliment for a Tweed Deluxe.

For the tube heaters:

6V6s = 0.45ma * 2 = 0.9A
12AX7s = 0.3ma * 2 = 0.6A

1.5A draw total. That looks okay.

For the 5Y3 you'd have 2A draw so that's okay as well.

Not knowing exactly what tubes you'd expect to use, you can assume the following as a starting point:

120Vac -> 370Vac -> Assuming 1.1 times increase and 40V drop from 5Y3 -> 365Vdc B+ voltage

Then you can design the power rail from there by chosing the proper dropping resistors for the B+_ voltages you need. I can hgelp you further once you decide on tubes and have a spec sheets.

Hope this helps!

martinman said:

I have the following Heyboer PT that is in need of a proejct:
Originally, I had this in a scratch build 5f1 but the plate voltage was way high (400+).

Click to expand...

Were you trying to use solid state rectification?

martinman said:

I couldn't bring it down with a bias resistor, so I ended up swapping it out for something closer to spec. Everywhere i look it says this PT is used for champs and princetons, but I just couldn't make it work in my build.

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Those amps are almost certainly spec'ing a 5Y3 tube rectifier. This transformer seems geared toward that.

Woah, nice!! This is helpful.

I was throwing out a 5e3 as an example because it's the next level up in the fender line. Ideally? I'd like to build a 5f1 / 6L6 variant.

Though, building something where I don't have to deviate from the original spec of the amp too much is the goal. So, whatever works -- doesn't even have to be a tweed.

I have both NOS 6v6 (RCA blackplates i think) or newer Tesla/JJ's. I can use either.

martinman said:

Woah, nice!! This is helpful.

I was throwing out a 5e3 as an example because it's the next level up in the fender line. Ideally? I'd like to build a 5f1 / 6L6 variant.

Though, building something where I don't have to deviate from the original spec of the amp too much is the goal. So, whatever works -- doesn't even have to be a tweed.

I have both NOS 6v6 (RCA blackplates i think) or newer Tesla/JJ's. I can use either.

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Assuming you have a suitable OT to support it, I would think that you could get away with a 6L6 Champ variant with that. You lose a 12AX7 in the transition from Deluxe to Champ so your current draw will be around:

6L6 = ~52ma for RCA Manual Design-Center Values
12AX7 = ~2ma

So definitely good for high voltage current draw.

Using a 5Y3 would probably put you right in the ballpark for a Class A 6L6 Tweed Champ.

You could basically just tweak the power rail from the 5F1 schematic to accommodate the 6L6 and change the cathode biasing resistor to something more appropriate. Screen resistors will be different as well but these are all minor tweaks.

Good news is that you should be able to use that PT for either a dual 6V6 based amp or a big bottle Champ. Keep in kind that you can play with the Tube Rectifier choice as well if you need more B+ voltage. Something with less voltage drop than the 5Y3.

If you decide to use SS rectification you'll have to drop the B+ pretty significantly even for the most robust big bottles. I can work through the power rail calculations depending on the rectifier type and tube choices you make.

Ok, let me head home today and I'll do some digging to see what I have running in the 5f1 i have now.

Out of curiosity... what about dual 6l6? Like a 5f4?

martinman said:

Ok, let me head home today and I'll do some digging to see what I have running in the 5f1 i have now.

Out of curiosity... what about dual 6l6? Like a 5f4?

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i don’t think that PT could support anything larger than a deluxe. 2 6L6s isn’t feasible, at least IMO away from the manuals right now, given the 60ma rating of that PT.

EpiLP1985 - Can you elaborate a bit on how you're thinking through this? I don't have a good grasp on how to evaluate what my options are and I really want to learn more about the hobby.

Maybe start with the the voltage, then move on to current as it relates to champ vs. deluxe (or more general: 1 6v6 vs 2 6v6 calculations). I just don't have a good enough grasp on this to really understand the fundamentals.

martinman said:

EpiLP1985 - Can you elaborate a bit on how you're thinking through this? I don't have a good grasp on how to evaluate what my options are and I really want to learn more about the hobby.

Maybe start with the the voltage, then move on to current as it relates to champ vs. deluxe (or more general: 1 6v6 vs 2 6v6 calculations). I just don't have a good enough grasp on this to really understand the fundamentals.

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No problem. For the record, I am not a subject matter expert here but my degree is in Electrical Engineering and I have a firm enough foundation in the basic principles that it translates into my hobby quite nicely.

IMHO, the first thing you want to do is establish what tubes you are going to use. This sets the tone for everything else. NOS tubes can generally handle a bit more voltage. They were built rugged, especially the JAN stuff, and thier data sheets are often conservative. That’s why you see the Fender amps of old really putting these tubes through what seems like extreme paces. Newer production tubes should be taken at face value according to thier data sheets.

Everything sort of starts with the power transformer. After picking tubes, the PT should be the first thing spec’d. It’s the power plant of the amp. Without getting in the weeds too much, I’ll speak in educated assumptions and some targeted simplifications to get the point across.

AC Mains voltage feeds the low side of the PT and is stepped up to its data sheet high side AC voltage. It is then fed through the chosen rectifier while simultaneously being stepped up and converted to DC. Depending on rectifier type, this can be anywhere between 1.1-1.4 times high side AC voltage. Keep in mind that solid state rectifiers will drop very little of the rectified voltage, whereas at the opposite extreme of this, the 5Y3 drops close to 40vdc. Different tube rectifiers bridge the gap between those two extremes. You can see how Fender got away with this high side voltages now in these little amps, huh?

Your power supply consists of X amount of filters and voltage reduction phases, i.e. capacitors and dropping resistors. They work in tandem to smooth the DC signal and reduce the B+ voltage (directly off the rectifier) to voltages suitable for the other stages in the circuits.

What’s important initially is determining the load on the PT from the various tubes. Power amp tube plates and screens draw a certain amount of current when operating. You would typically go to the data sheets for this info. We can hold off on actual calcs while we talk through the theory. Preamp tubes are similar except that plate currents will be the only current draw for triodes.

Current Draw on PT HV Supply = Power Tube Plate Currents + Power Tube Screen Currents + Preamp Tube Plate Currents

If this number < the quoted mA rating for the PT, then you are good to go.

Then you need to determine the load on the heater taps of the PT. This is the 6.3Vac taps on the high side of the transformer. These are rated in A as opposed to mA.

Current Draw on PT Heater Supply = Power Tube Heater Currents + Preamp Tube Heater Currents

If this number < the quoted A rating for the PT, then you are good to go.

Finally, if using a tube rectifier, you may need a 5Vac supply. I say maybe because a 9 pin rectifier like the EZ81 can run of the 6.3Vac supply, whereas most Octal rectifiers run of 5Vac.

Current Draw on PT 5Vac Supply = Rectifier Current Draw

If this number <.........

Finally, armed with all this, you can move on to determine which dropping resistors will give the required voltage to the power tubes plates and screens, as well as the preamp tube plates.

If thos finds you well well and it helps, we can go over some examples with actual calculations.

This... is brilliant. Thanks for taking the time to write it up.

Ok, so based on our discussion, it looks like 1 6L6 or 2 6v6 (w/ 1 12ax7, and 2 12ax7 respectively) will work. A couple questions on this. These are the 2 PT spec'ed for the 5f1 tweed champ, and 5e3 deluxe:


Mojo759 Champ
Mojo756 Deluxe

They are virtually identical save for the voltages. The current is the same too? Though in the Heyboer world, I found that their spec for the deluxe calls for almost double the current at 150mA.

What happens when the tubes are spec'ed beyond the PT spec? In my example above, what if I went forward with a 2 6l6 design with 3 12ax7? It would push beyond the spec and draw more current the available. Does this equal bad tone, or potential damage to the tubes?

Does the tone stack have anything todo with the amount of current draw? I mean, what if I wanted to build a treble, middle, bass, presence type of amp? There's alot going on with extra components. Does this have a big effect on the PT choice?

Sorry for the dumb questions... Also, apologies if I didn't use the right terminology .

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As far as spec for the amp, i think I'm stuck with the 5f1 chassis cut out as the 5e3 spec seems physically larger. I don't think the PT footprint will work with the 5e3 chassis I have in mind (and I don't have the facilities to laser cut anything), but I'll have to look into this more.


BTW, I measured my current setup -- i'm using a 5y3 w/ JJ 6v6 and 12ax7. It has ~400v on the plate for the 5f1. Before it was 450+ looking back at my notes using the PT mentioned above!

martinman said:

What happens when the tubes are spec'ed beyond the PT spec? In my example above, what if I went forward with a 2 6l6 design with 3 12ax7? It would push beyond the spec and draw more current the available. Does this equal bad tone, or potential damage to the tubes?

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Think about your house: Say you have a 15A breaker serving a typical sized room. That doesn't mean you are supplying 15A of current to that room all the time, but rather that you can accommodate 15A worth of current draw from something you plug in. What happens if you plug in something that draws 20A? Breaker pops, right? That's a safety thing and typically you are protecting the electrical components from being damaged when drawing more current than they can handle.

What if it wasn't there? Things like wire, receptacles, etc. would start to heat up. Go much past thier rating and the insulation on the wire and plastic on the receptacles might start to melt.

Now translate that to an amp. If you have a PT that is rated to safely operate for a current draw of 60mA and you overload it, it will start to heat up. You can feel it. Given how robust it is, you may even be able to overload the PT to a certain extent and as long as the unit doesnt get hot to the touch, you are probably good. Although you have to keep in mind that repeated overloading may have a cumulative effect on the PT's reliability and overall performance.

Does this translate to bad tone? If it affects the voltages delivered, maybe. Damage to tubes? If the PT blows it could have effects on downstream components, so yes.

martinman said:

Does the tone stack have anything todo with the amount of current draw? I mean, what if I wanted to build a treble, middle, bass, presence type of amp? There's alot going on with extra components. Does this have a big effect on the PT choice?

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Nope. The components drawing current on the various transformer taps are the tubes. Those things are associated with signal loss, etc. So I guess in a way they do add to the loading/current draw because extra tubes are typically added to drive them for signal recovery purposes.

martinman said:

Sorry for the dumb questions... Also, apologies if I didn't use the right terminology .

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No worries at all. Do not be confused here: I am no expert. I sometimes make calculation mistakes and terminology mistakes as well. I am still learning myself.

I'll post some calculations shortly.

Epi has a lot of sagely advice above :salude

My recommendation: sell the PT and get one that would fit your requirements. 370VAC @ 60mA (assuming you did not originally really mean 0.060mA) is not enough current to drive any serious output tubes. You could run a pair of 6v6 in AB as was mentioned, but even with only 2 preamp (1 for the PI), you're still nearing design limits on current. I like to add at least 20% margin of safety.

If you're set on keeping the PT, then my recommendation would be to build a tube preamp or similar device that does not use any significant audio output tubes. You could do something wacky like run a push-pull pair of 12AU7 for output on a tube preamp, and should be well within current limits with a low-wattage amp. Super low wattage non-master plexi?