Thunderstar Bass Heads........

59Panhead

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Decided to start a new thread, as I've got a few minutes this morning before heading out to spend the day with the wife.

I've got a couple Thunderstar Bass heads that have been sitting around for awhile. They are both 4-tube models, and I haven't dated either of them, yet. I will say that I'm intrigued by the design of these. A departure from the standard Fender/clone cookie cutter designs that are found everywhere.

Both were brought up on the lamp limiter, back when I first got them, though there was quite a time separation between as they were separate purchases. I have had the chassis out of both, as I like to do a visual before putting them on the limiter. One has the carry handle on top (big ol' handle, the same as on my Ampeg Fliptops), and one has the handle on one end.

The top handle model chassis and front panel are as a unit, and slide out the front. However, the end handle model chassis and front panel are separate, the front panel being held in place by machine screws on each end. It comes off first and then the chassis can slide out. Other than these differences, they are basically the same.

The top handle was robbed of all it's tubes, but fortunately I have an abundance of old stock. And, a few modern stock. I decided to go with modern on this one, as I don't use those unless I absolutely need to. I didn't need to this time, but I wanted to try them. Monitored the bias and adjusted, after a few minutes warmup. This amp has some minor hum issues, mostly from the pre-amp end. A small bit of power supply, but I'm ok with that for now. Overall, it is a great sounding amp.

The end handle was complete with old stock tubes, and needs major capacitor attention. Loud hum that not affected by the volume pot. Decoupling caps have to go, and then I'll troubleshoot from there.

I'll post my progress on them, when I am able to get to them. No pictures as of yet, but maybe this weekend. Have a good one.

Jack
 
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Cool stuff! Again, Thunderstars were Aaron Newman-designed and since he was an engineer, the design had really neat aspects to it. I appreciate your efforts as there isn't much out there on Guild amps.
 

59Panhead

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Fortunately, the schematic layout appears to be acccurate, though component values don't entirely match. I'm used to that on all my Gibson amps. The early GA-40 ('52-'54) schematic is virtually unreadable. I redrew that one and, with much input from others with better eyes, I was able to reconstruct an accurate rendition of it. That's when I found how much different my three of that model are. And, one of them is untouched, all original components. It doesn't work, though the other two are repaired and sound beautiful. I like having one in original condition for reference. I did give it a much needed cleaning.

So, I'll make comparisons of the non-working Thunderstar to a copy of the schematic, and mark it up as to differences. I probably won't CAD this drawing as the original is very clear and well made. We'll see how things go. Have a good one.

Jack
 

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I did not realize how long it has been since I posted this thread. These two amps have been on the back burner until a few weeks ago.

The amp with the top handle went on the bench a few weeks ago, and was given a close once over. As stated earlier in this thread, this amp was missing all the tubes. Love these 'non-working amps, for parts'. I have the parts. I mentioned some minor hum issues, but I'll address those in a minutes. I cleaned the cabinet and wiped down the chassis. What a nice amp. The pre-amp and the driver/PI have old stock tubes. I put Groove Tube 6L6GC's in the power amp. The noise lever is not bad, but I'm thinking now that I want to do some tube substitution to see if the noise changes. I will wait until the holidays to do that, as I have some time off.

The amp with the handle on the end turned out to be a problem child. Sunday (02Nov14), I took it to Church to show it to my amp and music buddy Don. He's our music leader, and says that my show-and-tell is a distraction for him. But, he never tells me to stop bringing such.

When I got home that afternoon, I figured to slip the chassis out, clean up the cab. Give the amp the once over, set the bias, and see how it worked. No such thing. After coming up on the lamp limiter, it had a loud hum that was unaffected by volume. My first suspicions was power supply caps.

First off, there was no bias current on V3 (6L6GC), and the current went out the ceiling on V4 (6L6GC). V4 began red-plating, so off came the power. I spent the early part of last week, troubleshooting. Discussing this with some friends, as I only has 30 minutes or so each evening to work on it, they were able to help troubleshoot the first issues with the V3 bias. The 6.8 ohm resistor was open on V3, and did not appear to be original to the amp. The one on V4 measured 11 ohms. On suggestions from my friends, I replace both with 1R - 3Watt resistors. Now, I had bias current on both tubes, but nowhere near low enough. In the 60mA range. Then, they both went off scale and both tubes began red-plating.

Out they came, and power back on, I began checking DC voltage on the control grids. Nothing out of the norm. -30+vDC, which didn't seem right, but I would get to that later. No high voltages, to indicate leaky coupling capacitors. Let me state here that I thought the power tubes were old stock, as they both had RCA on them. However, on closer inspection I saw 'Made in Russian' on each, and they look much newer than any of the old stock that I have. I don't know when RCA started having tubes made for themselves, but not knowing the history on these, I left them out and found some old stock to put in.

Now, the bias was adjustable down to a more comfortable level, though I ran out of range on one, a bit on the high side. I left them to see if they would maintain their bias. Also, let me mention that all noise went away, except for some low level hum. The Hum Balance cleared up most of that. This is when I decided to add an additional pot and resistor, to replace the 2.7K between the bias pots and signal ground. I found a hole near the OT and put a 10K pot in series with a 1.5K-1/2Watt resistor. I call that my bias level, and the other two, my bias balance. I am now able to bring the bias down to a level where I can truly balance both old tubes.

That's when another idea came to me. I have quite a few NOS Russian 6P3S-E tubes that I have considered to be 6L6G's. However, this article made me think different, giving a much better plate dissipation level that what I thought they should be:

http://www.jayskyler-guitarlessons-...ide/sovtek-5881-wxt-6l6-wgc-tube-guide-2.html

So, in with two of these tubes, bias to 60% of 25Watts dissipation (calculated off of my plate voltage for each tube), the amp got even quieter. Let it run for awhile, noting that the bias on each tube remained very steady, and only 1mA apart.

Connected my little Supro guitar and what a nice sounding amp. I connected my little Danelectro Cool Cat distortion pedal into the 'Pre-amp Output/Pwr-Amp Input' and verified that all was well there. This amp is better than the other. I may go back over the holidays and put a bias level control in it. Easy mod. And, put a couple of the 6P3S-E's in, for good measure, to see if that cleans up the hum a bit more. We'll see.

Anyway, here's a picture of both amps:



If I can see a way to post a couple PDF files, I'll attach my bias mod schematic and voltage chart. Take a look and tell me what you think. Have a good one.

Jack
 

59Panhead

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If someone can tell me how to post my bias modification schematic, please do so. It is a PDF file format.

And, the amp with the end handle is a '68 vintage. I'll date the other one, when I am able to slip it out of the cabinet, again, for it's bias mod.

Jack
 
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There isn't a direct way to post pdfs on the forum. Forgot where I originally saw your posts about this, but is the b+ still>500 volts? If it is, it might be that 8417s were the original tubes for that. They used a much lower bias voltage than 6L6s,maybe -15v iirc. If you are only able to get -30 max out of the bias, it was probably an 8417, originally.
 

59Panhead

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There isn't a direct way to post pdfs on the forum. Forgot where I originally saw your posts about this, but is the b+ still>500 volts? If it is, it might be that 8417s were the original tubes for that. They used a much lower bias voltage than 6L6s,maybe -15v iirc. If you are only able to get -30 max out of the bias, it was probably an 8417, originally.

Hoffman Amps is probably where you saw it.

No, B+ is some tamed, now, on the one amp that has the handle on the end. The B+ was going very high, as much as 534vDC, then settling out at around 500 or so. I put a thermistor on the Primary side of the PT, from my stash of Ampeg components. My filament voltage was jumping up to around 7.4-7.5vAC, and never dropping. Now, B+ never gets over about 490vDC, and the filament 6.8-6.9vAC, both ramping up very slowly. The thermistor is rated at 3 amps, right in line with the recommended fuse. I would prefer one rated a little higher, but I believe that it will be ok. I'm liking this soft start, as I'm sure the old caps, along with all the other components, are better off for it.

I don't remember what the other amp was running, but the next time that it's open, I'll make the same mods (bias and thermistor), and get a set of voltage readings.

Both amps are marked for 6L6GC's, and because of my mod to the bias supply, I can get just about any level that I need.

Jack
 

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Well crap. Another one followed me home. No history. It's been 'serviced', but it still doesn't work. Not my description. Just the seller's. Tubes glow, but no sound. I've gotten a few of these descriptions, in regards to head amps. No mention of speaker cab. Just the way I like it. Made them an offer, considerably lower, as there was 'no sound'. UPS put it on the front porch.

Maybe this weekend, I'll give it a good visual, inside and out. Check the tubes, and bring it up on the limiter. I can definitely see a bias mod happening to it, as the other one was easy. And, left the amp such that it can be reversed easy. No reason to do that, as any 6L6 (G, GC, 5881, etc.) can be biased for it. Unfortunately, pin 1 is being used as a tie point, so 7027's are out.

Anyway, I'll post progress and pictures, when any hands-on happens.

Jack
 

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Brought the latest jewel up on the lamp limiter (speaker connected). All tubes old stock. After it had some time, to observe for problems, I put it on full line voltage/current. The bias is a bit high, but still safe. Can't quite match the tubes, but they are both out of range that I want. This one calls for my bias level mod, to give the old tubes (or any, for that matter) a comfortable level to operate. Maybe I'll pull it apart Sunday, after Church, and start the process.

I plan to take one of the other ThunderStar Bass heads to Church, this Sunday, as we are having our Thanksgiving lunch. I usually carry something like this for entertainment, as there will be several musicians there to give it a workout. I have an Ampeg BSE 115T that works nicely with the ThunderStar.

Anyway, I'll post my progress with the latest, and what I find when I get it open. Have a good one.

Jack
 
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