6H100 Build Issues

[martin manning]

I don’t think you need to go thru having to make a bulb limiter, as your wiring seems to have been good all along, what I would do while ordering up the parts is to get some .200 to .250 amp fuses to use in place of that .500 amp fuse just for first time start up purposes.

These sound like famous last words, and anyway why waste fuses? Furthermore, you can observe the start-up transient in detail using a bulb limiter, especially if you use a bulb with a clear glass envelope.

If your building a limiter you can make it work for higher power amps when needed by just adding another bulb and socket and a 120 rated SPDT ( single pole double throw ) toggle switch and wiring it like this

You are showing a series combination of bulbs. You need to add the second bulb in parallel to increase the total wattage and max available current. Two 60W bulbs seems like a good choice.

[Stevem]

I must be thick because I can see why if I just had a one socket limiter and I could get it to handle a higher power amp by just installed a greater wattage build , then why with what I posted up is there a need to have a bulb in parallel?

[martin manning]

Suppose you want to use a 60W bulb for a 50W amp, and a 120W for a 100W amp, or work your way up from 60W to 120W. Two 60W in parallel will limit to the same current as one 120W.

[pdf64]

... I had a Bassman right in front of me with its standby switch wired in like his fuse in the PTs V+ center tap did not make for high voltages like he is having when the standby switch was open ,and I should have by what your last post is presuming!

There are several dozen different variants of Bassman designs, containing just about every possible permutation of valve amp circuitry. So for me to have any chance of helping you, please link to the appropriate schematic

OK, in the absence of that, I'll assume you've got an AA864; the same should apply to most models with a bias supply fed from a ~50V tap taken from two phase HT winding; specifically not a cathode biased 5B6 nor a 70W or other model with a separate bias winding.
https://el34world.com/charts/Schematics ... ematic.pdf
Modded such that its SPST standby switch has been moved from between the feed from the reservoir cap to the rest of the HT system, to the 0V return of the HT winding's CT.
Hence the HT line will be referenced to 0V via the 440k resistance of the reservoir caps' balancing resistors.
And with the standby switch open and the CT open circuit, a loose, high impedance 0V for the HT winding will exist via the bias supply, so effectively 355-0-245. A film reservoir cap then and open circuit at the HT rectifier output (ie as per the 6H100) would result in maybe 500VDC across the reservoir cap. However, the 440k balancing resistive load and various other leakage paths, in combination with the high impedance to 0V of the bias supply, will cripple that, such that there may only be a few VDC appearing in standby mode.

Compare that that the 6H100; as I explained previously

with CT open (and hence no current draw) the 0V reference would move to the HT leg connected to the bias supply, and the arrangement would resemble 680VAC at a very high source impedance, which would then be half wave rectified. Hence in that scenario, the VDC at the rectifier output can be much higher than normal.

Hence I think that if you arrange your Bassman's standby and HT & bias supplies as per the 6H100, and open the HT winding's CT return to 0V, you will be able to measure a very high VDC too.

[csjoyner]

While I’m waiting on the diodes, what volts would I need to see at post 3 of the power tubes to get to 100w? Also, how would I determine this?

[pdf64]

The query is a bit ambiguous / confusing, could you rephrase and expand? What exactly do you want to ascertain?

[csjoyner]

I beg to different, because there's no way with every wire off of that recto socket that the first filter node will have any impact on anything going on here!

Just note that it takes a bare bones minimum of 480 VDC on the plates of those two output tubes at idle to make just 60 watts rms, no less 100 watts of clean power once you start to Crank the amp up!

The voltages that he reported back about previously with the output tubes in and just idling where starting to sag just under that amount current load, no less playing thru the amp!

In short, that copper cap needs to be tossed out and that PT needs to be sent back as its defective!

One sure fire unquestionable way to prove out if the PT is at fault or not and end all of this Head scratching and Beard pulling is to disconnecting every wire, both primary and secondary, then pump in 120 volts off the wall outlet in to one red wire and the red/ yellow wire and then read the vac you get out of the primary wires, it will be less then the 120 volts your pumping in.

Next do the same thing with the other red wire wire and I bet your two readings will be vastly different which will then prove out that the PT IS BAD!

From what I can figure out by my math is that if both sides of the secondary winding where indeed good you would be reading 26 VAC on those primary wires assuming that you where pumping in 120 VAC, it's about a 4.5 step down factor it would seem.

I am referring to the second paragraph:
“ Just note that it takes a bare bones minimum of 480 VDC on the plates of those two output tubes at idle to make just 60 watts rms, no less 100 watts of clean power once you start to Crank the amp up!”

My question is what reading would you have to see on the plates to get to 100? Or am I missing something?

[pdf64]

The VDC at idle is not a good indicator of that. Under load, the HT may sag a bit or a lot, the degree can only really be found by testing it under load, ie at high power outputs. Also the OT impedance would need to be known.
The simplest and standard way to check output power is by connecting a resistive load to the secondary, feed in a suitable sine wave, and measure the VAC on the load at the onset of clipping.
Edit: ok the OT is 3k8, I suspect with that a pair of KT88 and an idle HT under 500V, 100W is not a realistic expectation.

[csjoyner]

The VDC at idle is not a good indicator of that. Under load, the HT may sag a bit or a lot, the degree can only really be found by testing it under load, ie at high power outputs. Also the OT impedance would need to be known.
The simplest and standard way to check output power is by connecting a resistive load to the secondary, feed in a suitable sine wave, and measure the VAC on the load at the onset of clipping.
Edit: ok the OT is 3k8, I suspect with that a pair of KT88 and an idle HT under 500V, 100W is not a realistic expectation.

What volts would make it realistic? I’m just looking for a goal of some kind to know we are headed in the right direction.

[Phil_S]

This is from the Genalex KT-88 data sheet. I guess 100W for a pair is possible under the specified conditions. Do you think today's tubes will stand up like the Genelex would have?

[pdf64]

What volts would make it realistic? I’m just looking for a goal of some kind to know we are headed in the right direction.

My experience with modern KT88 is that 100W from a pair is unrealistic; expect perhaps 80W from the 100w typical conditions.
In your amp, it may possibly get 70W.
That’s 1.5dB less that 100W, shock horror

[Stevem]

Power wise with today's tubes yes!
But I would feel a lot better doing that 100 watt rms output level with today's tubes if they where the Groovetubes/ JJ electronics version with the added riveted on plate structure to add mass.

I have had Marshall Majors on my work bench that output 238 watts of rms power out of its 4 KT-88 tubes.

They also burn up a lot of very very expensive output transformer too, lol!

[romberg]

While I’m waiting on the diodes, what volts would I need to see at post 3 of the power tubes to get to 100w? Also, how would I determine this?

The weber wpt100 power transformer's secondary windings put out 361Vrms relative to the center tap. Weber used to have a spec sheet for this where they sold transformers. But this one is no longer listed. So, I found the 361/361 by searching the web. It sounds about right.

Since this is an rms number you should expect your unloaded (no tubes) B+ voltage to be around 1.414 times this. Or 361 * 1.414 = 510 Vdc. This is an unloaded number and will drop when tubes are put in and drawing current. And due to part tolerances could vary 10-20%. It gives you an idea of what kind of B+ you should expect to see.

Mike

[Stevem]

A given output wattage can within a certain range of OT impedance be made by slightly different combinations of voltage and current minimums.

If you where for instance using a OT that could handle making 60 watts and was rated at 4.3k center tapped then you can figure out what the OT needs to drive it into either a 4 or 8 ohm speaker load,

Once you get all the math out of the way you can then see that it takes two output tubes that can put out a combined 240 ma and then a total voltage envelope of 500 volts.

The audio signal all on its own needs like 400 volts to drive this OT, but to keep the tubes from cutting off a minimum of 500 volts is needed when you do the math by the book.

On the other side of the coin 265 ma and less then 500 volts will get ya 60 watts also, but had better be sure the windings in the OT are rated for that much current!

Also if my math is in the ball park his 4 preamp tubes and the amps bias circuit draw vey little of that potential, like .03 amp tops out of that . 500 amp that his V+ is fused for.

[csjoyner]

Well, we got the diodes in and built the rectifier. While waiting we did go through and touch up some solder point and move some components around to make sure nothing was touching anything else. We also replaced the electrical tape with shrink tubing. Once we installed the rectifier we built the amp worked. All the knobs did what they were supposed to and the amp sounds great. Very Gilmour on Obscured sound. The bias test points read correctly and the voltage looks about what I would assume it would. Out of curiosity, we then reinstalled the copper cap again to see if it was indeed the new rectifier. It also worked and gave us the same voltage reading. So in conclusion, we do not know exactly what went wrong. We are guessing something was grounding out. Heck, maybe removing that electrical tape was what the amp really needed. Anyways, I thank you all. I learned a lot about building amps. I have included final pics (the 6H100 is the black amp on top of our last build the 6O100 orange amp) and voltage readings incase there is more to discuss. Next I have a Twin build I plan on revisiting. I will post a link to that here just in case any of you want to teach us some more. Again, thanks.

Copper Cap
4 - 334v
6 - 334v
8 - 443v

KT88 1
3 - 442v
4 - 434v
5 - -48.4v
Plate to cathode - 439

KT88 2
3 - 443v
4 - 436v
5 - -48.9v
Plate to cathode - 440

B+
A - 445v
B - 442v
C - 433v
D - 379v
E - 347v

Bias
to the Bias Pots - -72v
Based on the Plate to Cathode current we adjusted the bias pots to -59mv at the test point