SKYLINER build - inspired by ODS #124 and #060

[talbany]

Dumble used them in his (700V) Music Man amps

DSC00488.jpg

Tony

[martin manning]

Dumble used them in his (700V) Music Man amps

As did Music Man ;^)

[Stephen1966]

Dumble used them in his (700V) Music Man amps

DSC00488.jpg

Tony

Tony, Martin

I found a musicman schematic over at el34 world https://el34world.com/charts/Schematics ... matics.htm

Musicman (copy)_Page_01.jpg

I've highlighted the diode and cap shown in the photo. In this, they used a 500pF/3kV cap as well - MrD is emulating this in his amp - the photo is of MrD's amp right, or is it from the amp you built, Tony? The 500pF cap along with the 5kV/250mA diode seems a common theme up to the Musicman BB-3, GB-2, GD-2A and GP-3A where the cap is replaced by a 470pF/3kV. Interestingly, the diode stays the same, the only exception is the RD-50 which has been cloned here on TAG and there, I saw a cap, a resistor and a diode with reverse polarity to the cathodes of the tubes (which seems a less direct way of providing a load). In the Musicman RD-50, there is no cathode net and no diode on the plate/OT primary, only a 470pF/3kV cap on each of the OT primaries.

I also found the datasheet for the MR250-5 diode:

Book501-3485(1).pdf

It's probably a dumb pair of questions but what is the cap doing? And, how close does a modern diode need to be to the specs of the MR250-5. The closest I can find is the R5000F-B with a Rv of 5kV and If of 200mA. The other closest equivalent of the MR250-5 I can find is the 1N1734A but their prices are outrageous - between 17 and 24 USD each!

https://aerobasegroup.com/part-number/1 ... 0-947-5759

This is a modification i could do quite easily but, am I right in assuming I would only need to place the diode/cap net on one of the plates met by each primary? Even if one tube in the pair were to suddenly fail, it would surely be served by the net on the other in the pair.

Also, what's the story with that 1 ohm resistor in the photo??? A cathode resistor I assume. But 10W! I've heard of 1W, of 2W. I personally have 1 ohm/0.6W in mine...* perhaps I need to beef them up!? Also, it's huge! Is that mainly for reducing noise?

*Edit: Otherwise Pw = Ia squared x R

[Stephen1966]

My best guess on the cap to ground... low capacitance high frequency rejection during spikes/low impedance bypassing for lower frequencies. Basically, it blocks the spike. So what's the difference between a 500pF and a 470pF? Is it better to go lower?

Another thought, could this explain the cleaner, smoother "sound penalty"?

[bepone]

it is to reduce some highs..it is limiting tranformer bandwith. it is ok on some present amps.. also 1nF 10kV across the whole primary is ok too

[talbany]

the photo is of MrD's amp right, or is it from the amp you built, Tony?

It's a Mr D amp...192 (I think )Here is the other side..Pin's wired for 6L6's so diode and cap off the primary right to gnd (pin 8 )

Btw...Notice he puts a little dab of RTV at the base of the pin..Try and prevent any arching due to excessive fly back voltage along with the ceramic sockets

DSC00490.jpg

Tony

[talbany]

Digging a little deeper? The cap value appears to be a .001( 20% tolerance) Sprague ..As far as the 1 ohm cathodes? I personally would not go lower than a 2 watt there..Experience

DSC00454.jpg

Tony

[Stephen1966]

Okay, so after a couple of hours of surfing the dark corners of the internet I'm probably a little closer to understanding the role of the capacitor. Honestly, though, it's like wandering around a campus, and taking a few notes in one class before you realise it's not the right one, so you get up and go to the next class, only to find the same thing... and here I am two hours later, still grasping at straws

[The capacitor...]

it is to reduce some highs..it is limiting tranformer bandwith. it is ok on some present amps.. also 1nF 10kV across the whole primary is ok too

The closest I got to this was with this https://passive-components.eu/transform ... lications/. I don't understand the capacitor to be actually limiting the transformer bandwidth, quite the opposite: if anything it seems to be reducing the THD and increasing the reflected load. Thinking of the tranformer output gain as a horizontal line, distortion starts with the onset of return loss* at the knee at higher frequencies. The capacitor offsets the point where the return loss knee starts to decay to a point higher in frequency, flattening and broadening the output gain further into the inaudible frequency range.


*Return loss RL: Measure of the energy reflected back from the transformer due to imperfect impedance matching

2.43.jpg

ZS = source impedance; ZL = load impedance

At the very low and narrow range of frequencies we can hear, this offsetting of distortion seems hugely academic but by pushing the horizontal gain/clean signal further into the inaudible region and delaying the onset of distortion, would it perhaps be more accurate to think of this "reduction of highs" as more, a "reduction of distortion"?

Or am I simply barking up the wrong tree?

I still haven't found a clear answer about how much capacitance is good, whether less or more is better? None of the formulas I found seem to provide a means for the solution. But I'm not going to lose any sleep over it. A .001 ceramic cap will suffice, and we might challenge any ordinary human to tell the difference between this and a 500pF cap. [Blimey! That particular night at the tubeaholic convention was a real blast ]

Moving on, Tony's most recent photo doesn't show a similar diode/cap net on the adjacent tube in the 6L6 pair, so that answers another question. Vis: Can you use one net per pair? Or, do you only need to use one net for each leg of the OT primaries? Yes, you can. (I've noticed that people generally don't tend to post pictures of amps that failed) It is perhaps no coincidence that .001uF is exactly 2 x 500pF and we have two tubes in tow. So perhaps it is wise to think of this value, like the current it sees hitting the plate? And so, multiply the cap value, for the number of tubes it serves. Increase capacitance, decrease current. It's probably not helpful to think of current related to capacitance here except as a function of impedance, but as I said, I'm not worried about it, it just appears a corollary effect.

So, poor reader, realise that everything I've just said here has been cobbled together from random internet orifices. I would be happy, more than happy, for anyone with a real EE training and/or real amp building expertise to take me to task on these ideas and correct them.

For all the other people I've ever misunderstood, and all the girls I've loved... etc. - "Sorry!"

Onward and upward. Regarding 1 ohm sensing resistors power handling, okay, you've convinced me. It isn't so much about the power it has to handle during normal operations, rather what happens to it when there is a surge.

[bepone]

The closest I got to this was with this https://passive-components.eu/transform ... lications/. I don't understand the capacitor to be actually limiting the transformer bandwidth, quite the opposite:

check equivalent schematic of real transformer, and ask your self what is happening when one node you pull to the ground with capacitor? you are running all that reactances with internal tube resistance which is in kiloohms.. pentode is in several tens of kilohms..

[Stephen1966]

The closest I got to this was with this https://passive-components.eu/transform ... lications/. I don't understand the capacitor to be actually limiting the transformer bandwidth, quite the opposite:

check equivalent schematic of real transformer, and ask your self what is happening when one node you pull to the ground with capacitor? you are running all that reactances with internal tube resistance which is in kiloohms.. pentode is in several tens of kilohms..

You will have to elaborate for me to grasp the finer points here, but are you suggesting the cap provides a low impedance path to ground for higher frequencies; a low pass filter? Bearing in mind the effect of Return Loss, this seems counterintuitive. If I understand you well, you are saying the low pass filter effect is the dominant and more audible. Further, that the concept of the LPF might be used to determine the value of the cap and the roll-off of the high frequencies.

[bepone]

offcourse, without 470pF from anodes to the ground bandwith of preamp and power section (preamp is dominant) is maybe 20-30kHz roughly speaking without calculation, with 470F from anodes is maybe 15-20kHz.. i have somewhere some graphs..
see the bartel amp or tone king, they are using this often

[Stephen1966]

offcourse, without 470pF from anodes to the ground bandwith of preamp and power section (preamp is dominant) is maybe 20-30kHz roughly speaking without calculation, with 470F from anodes is maybe 15-20kHz.. i have somewhere some graphs..
see the bartel amp or tone king, they are using this often

Very interesting, I would like to see those graphs.

[FUCHSAUDIO]

Good to know, thanks. I'm just thinking what else I would need to do if I swap the PI plates' wires, maybe swap the plate resistors as well? Hmm. I'm hoping it doesn't come to that. The output section just after the PI is all but equivalent either side of the PI but the grid and cathodes are pretty well baked in to the circuit with half the stage fed by its grid and the other half by its cathode . Is this 50/50 because of the OT windings or the PT? It was Andy Fuchs who mentioned something about reversing the HV leads from the PT to remove oscillation. Is this not a better solution to the problem than re-engineering the LTP?

If the neighbours complain, I just send the wife out. She's in PR so it's just like homework for her

To be clear: My comment about reversing phase was for output transformer plate leads. If you have nfb, and the amp oscillates, you may actually have positive feedback, and the plate leads need to be swapped. Easy test is to lift the NFB wire from the speaker jack and if the amp gets louder, you have things correct and you have negative FB. You may need to look elsewhere if you have instability or problems with your output stage. If the leads are reversed, the amp would get quieter when you lifted the NFB wires and that shows you have positive feedback, which is wrong. I've seen amps work and sound okay, but "bark" or "tweet" when

Thank you Andy, I'm happy to stand corrected... Great tip about the volume with and without the NFB, but "bark" and "tweet"... when? We all love a cliffhanger but the suspense is killing me

Bark or tweet when pushed...guess I got distracted from finishing my post...lol.

[talbany]

.

It isn't so much about the power it has to handle during normal operations, rather what happens to it when there is a surge.

IMO.. You would be correct in assuming that

AFAIK..Peavey used the STR-2873 for their fly back diodes
https://www.tubeampdoctor.com/en/overvo ... eavey-amps
Another one would be R-3000F 3K/ 200mA
https://www.mouser.com/datasheet/2/345/ ... -14910.pdf
(BTW? They don't need to be FRED's, since they shouldn't normally be conducting)
As far as the cap goes? I always thought it was there to suppress runaway oscillations? but am curious to know otherwise

Here is a shot of the other sockets

DSC00453.jpg

Tony

[Stephen1966]

.

It isn't so much about the power it has to handle during normal operations, rather what happens to it when there is a surge.

IMO.. You would be correct in assuming that

AFAIK..Peavey used the STR-2873 for their fly back diodes
https://www.tubeampdoctor.com/en/overvo ... eavey-amps
(BTW? They don't need to be FRED's, since they shouldn't normally be conducting)

As far as the cap goes? I always thought it was there to suppress runaway oscillations? but am curious to know otherwise

Here is a shot of the other sockets
DSC00453.jpg

Tony

(Tony, I've put your link in tags - couldn't see it in your post...)

I'm inclined to go with the R5000F-B with a reverse voltage of 5kV, since if as rootz suggested, we see peaks over 1kV in an overdriven amp, and then when the load disappears that drives the voltage on one side even higher, 5kV appears to offer a much safer overhead for the diode. But if that were at the expense of less over-voltage protection for the OT i.e. thinking of the suppressed spike in 5kV diode as being higher than the suppressed spike in a 2kV diode then I could be convinced. 2kV just seems so much closer to the amps normal output in hard, overdriven performance. Really not sure. Is there any indication what MrD went for, he is sure to have considered all the factors. The R5000F has a forward current of 200mA but you have a good point, as it isn't meant to be conducting it is probably neither here or nor there.

I'm curious about the cap and oscillations as well... in my internet stumbling just today I came across a discussion that mentioned the cap with reference to feedback. Concentrating on the question of bandwidth of the transformer though, I didn't give it much attention and my short-term memory has already binned it. But now that you mention it, I'll add a +1

[Edit- A-ah! I see you edited your post to include the R3000F link... that's the same datasheet I'm looking at with the R5000F]