Sunn Beta Lead Repair - Voltages?

[BudJillett]

Hi All,

I'm wondering if there are any Sunn Beta Lead repair gurus in the forum?

I've attached a schematic I found in another thread on this site, which was said to be the equivalent of the Sunn Beta Lead, at least as far as the PA section.

My issue is the Amp will overheat and fry output components when the preamp is connected. This has only happened once and I fixed things to where I got the PA working again. However, if I play thru the preamp even for a minute, then shut off & feel the power resistors they get way too hot and I know a re-frying is only minutes away, if that.

However, if I bypass the preamp, I can play thru the power amp as long as I want.

I'm figuring something else has fried, something less obvious than the several components I replaced, and that something is on the preamp board. But that's just a guess as I have only hobby-level "training" in electronics.

Measuring the voltages where the preamp board connects to the PA board, I get a nice +14.9 volts on the appropriate pin, but a dubious (I think?) -13.1 volts on the pin that should measure -15VDC.

I'm thinking this should be a clue? Or is -13.1 in spec? (I tend to doubt it).

I'd be more than happy to provide additional information if needed. I'd love to get the preamp going again!

Thanks all,
Bud

[loudthud]

Is there DC on the output of the power amp when the preamp is connected?

There are a number of Non-Polarized capacitors is the signal path. One is C1 on the power amp board. These usually become leaky after 30 years of intermittent use. They all should be replaced. Most are 2.2uF, some are 10uF.

Check the collector Voltage on Q11 with and without the preamp connected. Q1 and Q11 really could use small heat sinks or chassis mounting, they run pretty warm.

[BudJillett]

Is there DC on the output of the power amp when the preamp is connected?

There are a number of Non-Polarized capacitors is the signal path. One is C1 on the power amp board. These usually become leaky after 30 years of intermittent use. They all should be replaced. Most are 2.2uF, some are 10uF.

Check the collector Voltage on Q11 with and without the preamp connected. Q1 and Q11 really could use small heat sinks or chassis mounting, they run pretty warm.

Thank you, loudthud, I really appreciate your help!

For testing purposes, I have the 14506 removed. I've pulled pin7 and then pin15 to ground with a 470-ohm resistor (though not at the same time). I did this with and without a guitar cable plugged into the FROM of Accessory Channel A. There doesn't seem to be much difference which pin is grounded, or whether I have a cable plugged in or not, the results are that no DC is measured at the speaker output.

The voltages at Q11 are:

C: -34 (though when I grounded pin15 this read -32 and -31.4 with two different readings -- only difference)
E: -13.1
B: -13.8

The -13.1 still seems too low to me, but I don't know. Maybe it's close enough to -15?

FYI, I've already replaced the 74C04 (with a C04). Added sockets for that and the 14506. I did put heat sinks on the TIPs. Every 4558 on the preamp board reads the same -13.1 at pin 4, as I guess they all would.

The 14506 was one of the components that blew out. I've "replaced" that with two other logic chips that I wired up according to a tip I found somewhere on the 'net. So, I have it -- it's just not in the socket at the moment (using the resistor for channel selection as noted).

On power down I do get a pop at the speaker, more-so in Channel A, and more-so if the preamp is connected. Not a horrendous pop. But a noteworthy one. In Channel B (RED) with the preamp bypassed I almost never get a pop. I will get a small one in that case if I turn the amp on then off too quickly. Otherwise that scenario seems pretty clean (other than there is still the dubious -13.1 volts at the emitter of Q11).

I removed 5 caps so far . . . a challenge on the double-sided board! Not looking forward to doing all of them. My soldering skills aren't the best and I'm getting paranoid I might damage something. You really think they're the main culprit at this point?


Thank you for the assistance,
Bud

[drewl]

When you are running the amp without the preamp are you driving it to as high an output as with the preamp?

It may be the power amp is failing when cranked.

Check to see if your bias is too high.
Check all emitter resistors to see if all output transistors are operating.
What replacement output devices are you using? There are better more modern beefier ones than the 3055's.

I work on a few of these owned by a band that just cranks the hell out of them into wrong impedances, as that's the sound they want!

[BudJillett]

When you are running the amp without the preamp are you driving it to as high an output as with the preamp?

It may be the power amp is failing when cranked.

Check to see if your bias is too high.
Check all emitter resistors to see if all output transistors are operating.
What replacement output devices are you using? There are better more modern beefier ones than the 3055's.

I work on a few of these owned by a band that just cranks the hell out of them into wrong impedances, as that's the sound they want!

Thank you, Drewl. I don't change the volume when I connect the preamp, so maybe you're right. The extra push is what's making the power amp run hotter.

I replaced the 3055s with 3055s. But I could not find SJ954s. I had one of those go bad and replaced it with a MJ2955 and left the other two as they seemed fine.

The TIP 29 & 30 I replaced with TIP 29A and TIP30A, which have higher voltage ratings. The Q1 circuit puts out a rock solid 15V, but the Q11 circuit gives me the iffy -13.1V. I replaced the power resistor and caps and zener in the Q11 circuit so . . . maybe the TIP29A expects a different bias setup than the TIP29? I wouldn't think so, esp. since the TIP30A circuit is working perfectly.

If I sever the -15V wire leading back to the preamp board and measure it, the voltage is still -13.1, so it's not anything on the preamp board pulling the voltage down. I may pop in another new zener just to see if maybe I put in a bad one, or maybe testing it compromised that zener somehow?

I did notice last night if I plug into "Accessory From A" I get pop on power down. If I plug into "Accessory From B" it powers down nicely. I plugged into "Master Accessory From" without selecting a channel and LOUD pop/buzz/hum on power on. Amp didn't like that at all.

What would you recommend in place of the 3055s and SJ954s? Maybe I'll just get all new devices off eBay and see if that fixes it.

Thanks for your input!

Peace,
Bud

[drewl]

Yeah, you see running into the loop or pwr amp inputs isn't driving the amp to full output.

It also sounds like your output devices are not matched so some may be working harder than others until they blow.

Get some newer higher powered devices like MJ15044's I think or similar, and the PNP match.
3055's are NPN
2955's are PNP

[BudJillett]

Get some newer higher powered devices like MJ15044's I think or similar

Thank you for the tip. Are those drop-in replacements? Or do you think I'd have to re-engineer the bias circuitry? (Hoping to avoid the latter).


Thank you,
Bud

[drewl]

Actually the part #'s are MJ15022 for NPN and MJ15023 for PNP.
forgot the actual part numbers.

Yes they are drop in replacement T03 style transistors with higher voltage and current rating.
Something like a few bucks each from Mouser or Digi-Key.

[BudJillett]

Actually the part #'s are MJ15022 for NPN and MJ15023 for PNP.

Thanks, Drewl,

A thread in another forum suggested to watch out for updated transistors, that because they can operate at higher frequencies that were not possible when the vintage equipment was first designed, unexpected self-oscillations could result. Do you (or does anyone) think those arguments have much merit? And, if so, should one be concerned in this specific case of the Sunn amp?

[drewl]

I haven't had any problems with that yet.
If you don't have an oscilloscope to see for yourself, listen for any odd distortion or if the output transistors get hot without playing through the amp.

[BudJillett]

Thanks Drewl,

I am going to order some parts for the Power Amp. I think it's fine after playing through it again tonight for quite a while. But it could be better and at the very least the output transistors should be matched.

I reconnected the preamp when I was done playing and ran some tests. There are definitely problems on the board. Some symptoms I'd forgotten, like REAL popping on power down and the channel selection still doesn't seem to want to work right.

I've been replacing the caps on it per Loudthud's suggestion, but haven't done 'em all yet. When the PA blew way back when, it took out the 14506 & 74c04, both of which I've replaced. But if it's not the caps (once I'm done) then I'm guessing something else in that area is less-obviously blown, or extremely compromised.

It was so much easier to find the bad components when they were crispy looking ;-)

--Bud