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Schematic help

BlacknWhite

BlacknWhite

Jun 22, 2012
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2,446
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Beersville, New Brunswick, Canada
equipboard.com
#1
Hey guys. After I picked up my YBA-1A it really peaked my interest into learning how tube amps work. I have been doing quite a bit of research on the components and how they all work. Safe practices... I'm now into the power section and signal path and trying to wrap my head around everything. I was wondering if some of the tube buffs here could look over the schematic I'm working on. There were a couple things I wasn't totally sure on but traced out what I though was correct. The 2 things that I wanted to make sure on was coming out of the first few amplification stages V2A I marked as a cathode follower into the tone stack. Then after the tone stack when it goes into V3A for further amplification... Plate from V3A goes to first output tube and the cathode of V3A goes to cathode of V3B to plate and then second out put tube (Phase Inverter)? That is where I was getting confused. Also anything else I have done incorrectly please point it out! Thanks in advance!
bassmastermkII.jpg
bassmastermkII.jpg
 
#4
okcrum said:
Yes, V2A is a cathode follower driving the tone stack. V3 is a differential (Schmitt or Schmidt) phase inverter with negative feedback applied from the secondary of the output transformer, and a presence (Hign Range Expander) control added.

Phase Splitters
Click to expand...

fdeck said:
I had to look up Schmidt phase inverter. Just rearranging the schematic makes it look just like the typical "differential pair" that's at the front end of op amps:

Understanding Audio

Very cool.
Click to expand...

Awesome. Thanks for that info guys!
 
#6
fdeck said:
Now I have to ask the experts: What's that diode-like thingy across the plates of the output tubes?

I believe this was the first real bass amp I ever played through, belonging to my high school, 1978.
Click to expand...

Thank you another question I meant to ask lol. The only thing I found that resembles it was a transient voltage suppression diode. I am reading up on it now and looking forward to the correct answer haha.
 
#8
That's the symbol for a tunnel diode, but even though they were commercially available at that time, I'm pretty sure that's not what it is because it wouldn't make sense.

What I would expect in that position (plate to plate) back to back zenar diodes which are related to TVS devices (which were probably not available at that time).

The purpose is to clamp flyback voltages that might damage the internal insulation of the output transformer.

There were lots of creative symbols developed for new devices before a standard symbol was agreed upon.
 
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#11
agedhorse said:
That's the symbol for a tunnel diode, but even though they were commercially available at that time, I'm pretty sure that's not what it is because it wouldn't make sense.

What I would expect in that position (plate to plate) back to back zenar diodes which are related to TVS devices (which were probably not available at that time).

The purpose is to clamp flyback voltages that might damage the internal insulation of the output transformer.

There were lots of creative symbols developed for new devices before a standard symbol was agreed upon.
Click to expand...

I found a post that says the device was called a thyrector and at some point they became obsolete and Traynor advised clipping them out of the amp if the went bad.
Repair Question - Traynor YBA 1 - looks like a cap

Here are a couple of articles describing the thyrector.
DatWiki.net - Aviation Dictionary Presented by Aviation Supplies and Academics, Inc.
Invalid Link Removed

My Trace Elliot series I V-Types and Ashdown tube amps uses multiple reverse biased 1N4007 diodes connected in series from the outer winding of the OT to ground and I believe the intent is the same. Spec for 1N4007 says the reverse voltage is 700V and the Max Peak Repetitive Reverse Voltage is 1000V. The TE amps have two 1N4007s from series to ground on each side of the OT. The Ashdowns have three. This method is mentioned in the thread I linked earlier in this post as well.
 
#12
Varistors have been around since the 1920s, made out of various things like copper oxide, selenium, and even silicon carbide. Think early rectifier technology. The common ones we see today and refer to as MOVs are mainly zinc oxide on a ceramic substrate. They all act like back-to-back zener diodes. They've had 3 or 4 different names and symbols over the years. The old Bell telephone system and telephones were full of them.

The advantages over diodes are that they are nonpolarized, clamp faster, and generally have higher peak current ratings. Both fail spectacularly when subjected to much greater than their ratings. An engineer I had the misfortune to work with once specified 480VAC MOVs for a large power system at a Portland cement plant, without checking what the actual primary power supply voltage was. It turned out to be 575VAC and tripped the entire plant offline when energized, as well as temporarily blinding everyone standing in the power vault by the flash through the seams in the cabinet doors. :rollno: I had to drive up next day with a car full of parts and tools to get the damn plant back online.

You could replace the one in that Traynor with any Link Removed like a Littlefuse V625LA80CP rated at 600+ VAC. Peak voltage handing is then at least 848.5,V, matching the original part. FWIW, you can still buy selenium varistors today.
 
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#13
Interesting, that's a selenium type device which has been obsolete for decades. That symbol was GE's own symbol for what with some modification eventually became a generic symbol for a TVS type device.

They are functionally like a back to back connected Zener diodes.

Replacing with 1N4007 diodes is not the same and will not protect the same. TVS devices operate like symmetrical clamps, diodes operate asymmetrically (VERY much so)

A 600V RMS rated MOV might be a closer choice though I believe there are solid state TVS devices that are even more similar. There are also specialty Zener diode devices available.
 
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#14
That will be a thyrector diode which protects the plates and the output transformer. I haven't seen one since the 80s. The old ones will look like an old axial capacitor that is wrapped in cardboard, like a mini tube from toilet paper.
 
#15
agedhorse said:
Replacing with 1N4007 diodes is not the same and will not protect the same. TVS devices operate like symmetrical clamps, diodes operate asymmetrically (VERY much so)
Click to expand...

I do understand diodes operate asymmetrically. However in the given configuration It would seem they would offer symmetrical protection. Voltage spikes that exceed the reverse breakover voltage of the diodes are just shunted to ground instead of across the primary. What am I missing?

Here is the diode protection scheme
upload_2020-1-6_14-1-36.png

and here is the thyrector protection scheme.
upload_2020-1-6_14-2-28.png
 
#17
Wasnex said:
I do understand diodes operate asymmetrically. However in the given configuration It would seem they would offer symmetrical protection. Voltage spikes that exceed the reverse breakover voltage of the diodes are just shunted to ground instead of across the primary. What am I missing?

Here is the diode protection scheme
View attachment 3657597
and here is the thyrector protection scheme.
View attachment 3657599
Click to expand...
I believe the function of the diode protection scheme is not for the diodes to start conducting if the voltage becomes excessive - it's to prevent a situation in which the voltage goes negative because of impedance mismatch.

Consider what is going on in the output tubes and transformer when it is trying to develop high power into too high an output impedance. Current through one output tube is decreasing, the other is at high current. The voltage at the transformer primary is high on the first tube (with low current flowing) and low at the second tube conducting heavily. The mutual inductance of the two halves of the primary forces the voltages to be equal magnitude but opposite polarity relative to the center tap. For AC purposes, the center tap is grounded (the DC voltage is high, but constant).

Eventually the decreased current draw on the first tube will force the opposite side of the primary (i.e. the voltage at the plate of the tube conducting heavily) to go negative. At this point, the tube that was conducting heavily shuts off suddenly (it's acting like a diode). The energy stored in the magnetic field produced by that current has to go somewhere! The voltage across the primary spikes to try to keep current flowing - this is the "flyback" condition that causes arcing. The protection diodes give that energy a place to go by allowing the current to continue to flow (shorting the spike to ground).

If the impedance is matched better, the first tube cuts off while the second is still conducting. The second tube's plate voltage remains positive, that tube continues to conduct, and negative feedback in the power amp forces that tube to do all the work. With the impedance matched properly, this all happens while the second output tube is still n a range where it is capable of operating linearly. This is class AB. And it's why impedance matching is so important for tube amps.

Edit: minor clarification.
 
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#19
agedhorse said:
Interesting, that's a selenium type device which has been obsolete for decades. That symbol was GE's own symbol for what with some modification eventually became a generic symbol for a TVS type device.

They are functionally like a back to back connected Zener diodes.

Replacing with 1N4007 diodes is not the same and will not protect the same. TVS devices operate like symmetrical clamps, diodes operate asymmetrically (VERY much so)

A 600V RMS rated MOV might be a closer choice though I believe there are solid state TVS devices that are even more similar. There are also specialty Zener diode devices available.
Click to expand...

Yes, A TVS (thyrector) could do the job too. Ultimately, the best, fastest, most survivable protection would be a reliable Invalid Link Removed, but those are not available for general use because of their construction, and the fact that they have other uses as ultrafast switching devices.
 

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