new tube amp!

[megadan]

So I got a nice little (big!) package from the UPS guy yesterday.

It's an old Guild Thunderbass... wow, it's a lot bigger than I thought.

pictures coming soon!

Here's what I know so far

I opened it up, before and after trying it out (more on that in a sec). It's very old! and dirty inside... but that can be cleaned... very simple... 6 tubes, 3 caps, and 2 transformers. Someone was good enough to label the tubes, too. Right now it has 2 6550's (actually a "6550" and a "6550A") in the power tube section, plus a 12AU7A and what appears to be 2 12AX7's... that took a bit to figure out... one of the 3 preamp tubes was labeled "7025", which I found out is just another name for a 12AX7... the other tube wasn't labeled... but it looks the same (I think), so it might be another of the same... (or maybe it's a second 12au7a?)

And then the weirest one was, there was another power amp tube (just one), a 7591A. I couldn't find nearly as much on that one, just that EH makes one. I'm not sure why it's there (as preamp or power?) or, if it's power, why there's only one (don't they usually come in twos?), or, if it's preamp (which is the "section" it's in), why is there a power amp tube in the preamp section (I surmise it being a power tube because it's sold in matching pairs, from what I've seen).

Whew.

edit: research indicates that the 7591 is a rectifier tube... it seems like that has something to do with the power supply, but I'm not quite sure...

 

 

it also has some neat controls on the front. Looks like there are two channels, each with two inputs. One channel is "Bass," the other is "Normal."

There's a volume, treble, and bass knob on each one, and a 3 way slider switch that has diagrams: a solid "O", an outline "O" and two "O"' in a bullseye config... I haven't quite figured that out, but I think it's a tone thing.

 

It also has a 3 way power switch (On-/Off/On+), and a standyby switch. And, a very large master volume knob.

 

It has controls on the back for "hum balance", a dampening switch (high / low), 2 bias adjust screws, and 2 speaker outputs (speaker, and ext. speaker, both 8 ohms).

 

I think that's all the features. Oh, and a two prong plug, which I learn about in a moment!

 

 

So luckily, I had practice that night, cause I wanted to see if it worked, of course. So I took it with me, and hooked it up to my cab (the big ben). I checked all the knobs, connections, plugged it on, set the switch to standby, and crossed my fingers. Then I flipped it on. Viola, it light up. Good first step. I know I should wait to let the tubes warm up. I was pretty anxious, but I managed to wait about 30 - 45 seconds, before I flipped the standby switch. It gave a nice good chunk inthe speaker (or was that when I turned it on?). I adjusted the knobs, and voila, bass noise! It had volume, I was impressed! It worked! I played with the knobs a bit... I turned it up... it was plenty loud! Some of the switchs a little scratchy, but that was to be expected.

I'm afraid I may have pushed it a little hard right off the bat though... but it was just starting to sound sweet as I spun those dials upward!

I had plugged into the "bass" channel, so I flipped it over to standby, unplugged by bass, and plugged into the normal channel.

 

But, when I went to turn it back on, it wouldn't come out of standby. Still light on the power, but no signal at all. I looked at the power tubes. They were glowing, quite hot, but it looked like the first one was now glowing quite a bit less than the other. Did I mention the great burning smell? Ha! yeah, it was pretty dirty, it smelled like burning dust.

 

Anyway, it wouldn't come back on, so I went to turn it off, and it have me a nasty shock!

So I swatted at the power switch with the neck of my bass, haha... finally flipped it off, and unplugged it...

 

Whew.. lengthy story...

how about some stupid questions?

First some thoughts... I will defintly need to get some work done to it. First on the list is swapping the 2 prong for a grounded 3 prong cord. Safety first!

I also pulled out the tubes and had a look at them. I think it needs new power tubes. The preamp tubes look ok (Well, I'm sure they need to be replaced too, they just "look" better than the power tubes because they deal with less power).. but the power tubes are defintely burnt on the sides.

But yeah, does that sound like a fried power tube to you? No output, very little glow in one tube... that seems like the common sense diagnostic, but I don't know.

 

Ok, now the dumb questions!

 

Do preamp tubes have to be put in exacly the right way, ie, the pins lined up in the exact configuration in the slot? Or does it matter? I noticed with one of the power tubes that it had a guide in the middle, so it could only go in one way. The other one did not have that (but I carefully observed it's orientation so I could put it back the same way)

 

Is it likely that my mystery tube is just another 12AX7? or...?

(pictures coming soon!!)

 

will running the amp (at least for a short time) with a mismatched load hurt it? I've heard from many sources that tube amps will tolerate a mismatch of up to 2:1 in either direction. I've only worked with SS power sections before, so...

 

If I wanted to, could I do something like this: plug bass into bass channel, input 1. Run short patch chord from input 2 to input one of normal channel, thus using both channels at once. Does this give a) different tone or b) more gain/overdrive?

 

Can I replaced the 6550 and 6550A with a set of regular EH 6550's?

 

What exacly is a rectifier tube for?

 

Any other tips on coming to the tube side?

[megadan]

sorry, that was really long, I know

[oldivor]

suonds like what happened to my V-4B. After 30 minutes of playing mine it flashed out light from the back had a smokey smell. All it needed was new tubes. I'd check to make sure all teh tubes are in thier sockets too.

 

And EH tubes are what Ron l Hoover says are tyeh best new tubes IRC. And since he build tube amps(I think HiFi stereos) I trust his word on stuff. And adding a ground. Me and my dad adding a ground to my V-4B in about 3 hours. I'm sure if you knew what you are doing you could do it easily.

[speddling]

Killer Find!

 

I spent a long time researching the 6550/KT88 family of tubes...

 

I ended up with Sovtek KT88 and I'll buy them again in a second if I get another amp that uses this type of power tube!

 

The EH stuff is what I went with for the 12xx7 stuff.... well actually I got 2 barely used NOS JAN 12AX7 and a JJ & a EH 12AX7 real cheap so I only needed to buy one 12AX7 and one 12AT7

 

run any oddball tubes past Christian @ http://www.tubedepot.com

 

He's a great guy and has a great selection of old hard to find stuff

 

FYI: the 7591 is a power tube... might be used as a tube driver in that amp.

http://tubedepot.com/eh-7591a.html

the pricing is a matched pair... I'm sure Chris would sell you just one.

[speddling]

and disclaimer time.....

 

careful where you stick your hands and tools.... touch the right thing at the right time and it will kill you!

 

and not "Oh man that kills"...

 

I mean your family dressed in black and you on displayin a box wearing a backless tux!!!!

[nosamiam]

 

Originally posted by speddling and disclaimer time..... careful where you stick your hands and tools.... touch the right thing at the right time and it will kill you! and not "Oh man that kills"... I mean your family dressed in black and you on displayin a box wearing a backless tux!!!!

 

 

Very true! Even after being unplugged and sitting on a shelf for months. I'm NOT joking. Capacitors, by nature, store charge in them, even after the unit is unplugged. If you get current from one of those caps going through your body, it could at the very least burn you badly and could kill you quite easily. If you must poke around in there without discharging the caps first, only use one hand.

 

Better yet, build a cheap, quick, and easy capacitor discharger. It's just some alligator clips, some wire, and a wire-wound resistor. Takes about 5 minutes to make.

[der oxenrig]

Hooray for mismatched powertubes! My SB-12 has a G.E. 7868 in one slot, and some weird Sylvania tube in the other! Sounds like total ass!!!

[oldivor]

Originally posted by The Ox Hooray for mismatched powertubes! My SB-12 has a G.E. 7868 in one slot, and some weird Sylvania tube in the other! Sounds like total ass!!!

 

Your SB-12?

[der oxenrig]

Originally posted by mxpxfan Your SB-12?

 

[oldivor]

Originally posted by The Ox

 

hhmmmm

 

YOUR SB-12

[der oxenrig]

Originally posted by mxpxfan hhmmmm YOUR SB-12

 

well, at least for a year.

[oldivor]

Originally posted by The Ox well, at least for a year.

 

 

Thought so

[ZigmundFloyd]

Originally posted by The Ox Hooray for mismatched powertubes! My SB-12 has a G.E. 7868 in one slot, and some weird Sylvania tube in the other! Sounds like total ass!!!

 

I want that amp!!

[megadan]

 

 

 

 

[der oxenrig]

Originally posted by megadan

 

 

Mmm, lots of old treasures in that amp!!!

[oldivor]

Originally posted by The Ox Mmm, lots of old treasures in that amp!!!

 

+1

[bassman1956]

Retube it, then sell the old tubes on eBay as vintage collectables!

[bassman1956]

While we're all drooling about tube amps, a question: Why is it common to mount the tube chassis on top, and have the tubes hanging down underneath? Heat?

[megadan]

I have some more "tube shots"

 

 

 

So what of these tubes are "valuable" or at least "treasures"?

 

There's an RCA 7025, a Mullard 12AU7X (that one?), a looks like GE 7025/12AX7 (that ones all worn off), and a Sylvania 7591A (and that one?), an RCA 6550, and a (GE?) 6550.

Whew.

 

 

I habe no quasm retubing it and selling 'em on the bay if that would actually be worth anything

[megadan]

Here's the "mystery tube":

 

 

Here's the back panel (and no tubes):

 

 

 

ID's:

 

 

 

Swapped the power tubes around to test by hypothesis of bad power tube, and rule out a bad socket:

 

 

 

Exciting...

 

Assuming my hypothesis is right, I'll need to order some new power tubes.... 2 x 6550A or 2 x 6550?

Would a pair of these hit the spot? http://store.tubedepot.com/eh-6550.html

 

Oh yeah, and any one have experiance w/ the many tube stores shipping to Canada?

[oldivor]

*drool*

 

hhmmm RCA

[megadan]

RCA, eh?

 

Looks like it's the RCA 6550 that's the bad tube, I just tried them swapped around, and the GE 6550A light up real good like, but nothing from the RCA one.

[speddling]

the GE 6550a is the Holy Grail tube for SVTs and Fender 400PS amps... if you can find one that 'matches' it, that'll be a nice pair for the power amp section.

 

all the 12**7 tubes are just used old USA tubes.... fully tested good they run about $7 to $12 a piece.

 

I'd get a full set of EH (though I'd get the Sovtek kt88 again) from a tube dealer that tests and matches tubes.

[rambler1959]

you dont want to run a tube amp at a mismatched impedance at all!!! solid state amps can usualy be flexible with this but tube amps can melt down from it.

 

(although some 4 ohm amps can safely run 8 ohm loads, the older bassmans come to mind, im sure its not recomended)

[L. Ron Hoover]

Originally posted by nosamiam Very true! Even after being unplugged and sitting on a shelf for months. I'm NOT joking.

 

Nah, this is a bit of a myth. If the caps are actually in a circuit they'll eventually bleed off, it might take 10 or 15 minutes but they'll drain. Properly designed amps wil have drain resistors on the big caps anyway.

 

Now if you charge a cap up and leave the terminals open, it'll hold the charge for quite a long time.

 

The DC voltages inside most tube amps most likely won't kill you, but they'll give you a good root. I can tell you this from experience. The really high voltage ones, like SVTs (650V) are obviously more dangerous. In general, you're more likely to be injured or killed by the 120V AC mains than by any DC voltage in a tube amp.

 

 

DISCLAIMER:

 

This does not mean that you should be poking around inside an amp if you don't know what you're doing!!!!

 

 

Megadan: My advice is that you bring this amp to Brit Fader in Cole Harbour. Let him give it a once-over He's probably the best tube amp tech in the country. It looks like it might need some TLC.