NAD: Thunderstar Bass Amp

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
If you run the line out of the transistor Guild into the preamp in, does it get both loud and awful at the same time?

You mean the model 4 amp? I did that with the Micro Cube yesterday to test that the power amp section was working. I'm afraid to try it with the little Guild because it might sound good and invalidate my inebriated ranting. :eagerness:

BTW I was making myself a cocktail because I need a break from thinking and my wife asked if I was going to work on the amp some ore. I replied that no, amps are like handguns: you don't work on them when alcohol is involved.
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
I might have to punt this one to an expert. Neither schematic looks right to me. This amp has two 12AX7s and to me it looks like both triodes are in use on both of them which is not reflected on either schematic. I can trace a schematic and I've got all great tools, but as I've said before, I'm no EE, and I think I've gotten to the limit of my knowledge and abilities when it comes to amps. What's killing me is that there's nothing visually wrong, and the amp worked great before I cleaned it. The only thing that happened was the volume seized, and that moves freely now, but what I'm seeing isn't reflected in either schematic so I don't know if it's right.

Also the tone switch just hurts my head no matter which schematic I look at.

Default, you want a whack at it? I know you're crazy busy but I'm happy to drive it at least close to you.
 

SFIV1967

Venerated Member
Joined
Nov 20, 2010
Messages
18,442
Reaction score
8,956
Location
Bavaria / Germany
Guild Total
8
That 7-29-69 schematics in post #46 is not for the amp you got. That is only a 4 tube amp but you got a 5 tube one.

The 1-11-71 one looks like what you have (the volume pot in your amp is dated 7032), it shows both the front and rear volume pots and it shows both 12AX7s for V1A and V1B and also as V2A and V2B!
V3 is the 7247. Seems all to fit as far as I can see.
In that schematics the volume pots are both 1M and not 500k/1M as you measure. Maybe that one volume pot is still screwed up in your amp.
Here is the link: http://www.prowessamplifiers.com/schematics/images/Guild Tbass 2 tube preamp.pdf

Ralf
 
Last edited:

AcornHouse

Venerated Member
Joined
May 22, 2011
Messages
10,213
Reaction score
7,209
Location
Bidwell, OH
Guild Total
21
I might have to punt this one to an expert. Neither schematic looks right to me. This amp has two 12AX7s and to me it looks like both triodes are in use on both of them which is not reflected on either schematic. I can trace a schematic and I've got all great tools, but as I've said before, I'm no EE, and I think I've gotten to the limit of my knowledge and abilities when it comes to amps. What's killing me is that there's nothing visually wrong, and the amp worked great before I cleaned it. The only thing that happened was the volume seized, and that moves freely now, but what I'm seeing isn't reflected in either schematic so I don't know if it's right.

Also the tone switch just hurts my head no matter which schematic I look at.

Default, you want a whack at it? I know you're crazy busy but I'm happy to drive it at least close to you.
Par for the course when you’re talking Guild amps. They were constantly changing and there’s usually only one version available as a schematic (vs. Fender, where you can find schematics for the 5a3, 5b3, 5c3, 5d3, 5e3, etc... Deluxe.)
 

Default

Super Moderator
Platinum Supporting
Joined
Jul 30, 2007
Messages
13,596
Reaction score
3,008
Location
Philly, or thereabouts
Guild Total
11
I might have to punt this one to an expert. Neither schematic looks right to me. This amp has two 12AX7s and to me it looks like both triodes are in use on both of them which is not reflected on either schematic. I can trace a schematic and I've got all great tools, but as I've said before, I'm no EE, and I think I've gotten to the limit of my knowledge and abilities when it comes to amps. What's killing me is that there's nothing visually wrong, and the amp worked great before I cleaned it. The only thing that happened was the volume seized, and that moves freely now, but what I'm seeing isn't reflected in either schematic so I don't know if it's right.

Also the tone switch just hurts my head no matter which schematic I look at.

Default, you want a whack at it? I know you're crazy busy but I'm happy to drive it at least close to you.

Sure, things are lightening up at work for a couple of months. I'll lend you a review amp, if you'd like.
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
That 7-29-69 schematics in post #46 is not for the amp you got. That is only a 4 tube amp but you got a 5 tube one.

The 1-11-71 one looks like what you have, it shows both the front and rear volume pots and it shows both 12AX7s for V1A and V1B and also as V2A and V2B. V3 is the 7247. Seems all to fit as far as I can see.
Here is the link: http://www.prowessamplifiers.com/schematics/images/Guild Tbass 2 tube preamp.pdf

Ralf


GAAHH!!!! That's SO much better - thank you!

What motivates people to plaster their own watermark over information that they don't own? That's infuriating. Here, I fixed it so I can actually use it.


Thunderstar-Bass-schematic-Jan-1971-clean.png
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
Sure, things are lightening up at work for a couple of months. I'll lend you a review amp, if you'd like.

Nice. Now that I have what appears to be a correct schematic I'll take another shot at it, but after tomorrow I have a book to edit so I'll need to take a step back, though a review amp might be a nice distraction from that! :congratulatory:
 

SFIV1967

Venerated Member
Joined
Nov 20, 2010
Messages
18,442
Reaction score
8,956
Location
Bavaria / Germany
Guild Total
8
According to the schematic the rear volume pot has a linear taper and the front volume pot a 10% log taper. Interesting little details...
Ralf
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
According to the schematic the rear volume pot has a linear taper and the front volume pot a 10% log taper. Interesting little details...
Ralf

I saw that. That means I'll definitely never find one of those as a replacement. I did find a video showing how to make your own by cannibalizing two dual pots, so that's useful.

Mu gut tells me that it's gotta be that pot because I hadn't touched anything else and that had given me trouble.
 

fronobulax

Bassist, GAD and the Hot Mess Mods
Joined
May 3, 2007
Messages
24,708
Reaction score
8,836
Location
Central Virginia, USA
Guild Total
5
What motivates people to plaster their own watermark over information that they don't own?

They might actually own it or be taking steps to do so. I know of several books where all of the content was previously published in books, newspapers and government publications at least 100 years ago. The books are copyrighted and the copyright has been upheld. The intellectual property is in curating the collection.
 

SFIV1967

Venerated Member
Joined
Nov 20, 2010
Messages
18,442
Reaction score
8,956
Location
Bavaria / Germany
Guild Total
8
I saw that. That means I'll definitely never find one of those as a replacement.
The good capnjuan already gave up on that in the past! He wrote: "The chin-scratching here has to do with whether the replacement should have audio a/k/a logrithmic taper or linear taper. It's just a guess but I think the design anticipates feeding signal to the back half of the 7247 in a 'linear' manner; that is, for each partial rotation of the shaft, there is a proportional increase in signal strength (decrease in resistance to ground). This might have been designed to keep the driver and outputs from bogging down.

Audio taper works a little differently where the change in resistance to ground for each partial shaft rotation is non-linear; that is, a little bit of shaft rotation results in a greater change in resistance to ground. Most audio volume controls are 'audio taper'. If the original was both; 1/2 audio taper and the other half linear taper, the market no longer supports a control like that; it's either audio or linear for both parts of the control and the question is, if forced to chose one or the other, which one is 'better' for the amp or screws it up the least. Right now, I don't think I have an answer but we have several solder-heads lurking and maybe one of them will have something to offer on the subject. cj
"

But who was the manufacturer of the dual pot in your amp? Can't see the manufacturer number in your picture except the Guild part number 004027 and 7032 datecode.

Ralf
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
They might actually own it or be taking steps to do so. I know of several books where all of the content was previously published in books, newspapers and government publications at least 100 years ago. The books are copyrighted and the copyright has been upheld. The intellectual property is in curating the collection.

If I can validate that as being the case I will gladly give them credit. I have a feeling that’s not the case, though. Much like the people who have scanned Guild catalogs and charge to let you download them, my gut says this is what’s happening.

I get it - Capatalism! But when there’s no legal ownership of the material I find the practice reprehensible. As always I’m happy to be proven wrong.
 

fronobulax

Bassist, GAD and the Hot Mess Mods
Joined
May 3, 2007
Messages
24,708
Reaction score
8,836
Location
Central Virginia, USA
Guild Total
5
If I can validate that as being the case I will gladly give them credit. I have a feeling that’s not the case, though. Much like the people who have scanned Guild catalogs and charge to let you download them, my gut says this is what’s happening.

I get it - Capatalism! But when there’s no legal ownership of the material I find the practice reprehensible. As always I’m happy to be proven wrong.

I agree with you that I don't like it, but sometimes I just have to justify the charge as paying for their time to find and scan it. Of course that is probably a couple orders of magnitude more than minimum wage but...
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
Oscilloscope time! This is all just signal trace.

Question - is it normal for the voltage to drop so much after the tone controls?

ThunderstarBassVoltages.jpg


The (#16) stuff means I have a screenshot of that point. Here's #16, for example:

TStarBassTroubleshoot-16.png
 

SFIV1967

Venerated Member
Joined
Nov 20, 2010
Messages
18,442
Reaction score
8,956
Location
Bavaria / Germany
Guild Total
8
Question - is it normal for the voltage to drop so much after the tone controls?
That's indeed odd that this trebble pot in 10 position would drop the voltage from 7.68V (#6) to 1.32V (#7). Let's see what Steve has to say. The good thing, if this pot would be the source of trouble a replacement would be no problem.
Ralf
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
But who was the manufacturer of the dual pot in your amp? Can't see the manufacturer number in your picture except the Guild part number 004027 and 7032 datecode.

Ralf

304 7032

Stackpole Electronics, Inc.
in the 32nd week of 1970
 

GAD

Reverential Morlock
Über-Morlock
Joined
Feb 11, 2009
Messages
22,583
Reaction score
17,800
Location
NJ (The nice part)
Guild Total
112
Added DC tube pin voltages.

Yeah, swapping out either the bass or treble pots would be a joy compared with the volume or anything having to do with that crazy tone switch.
 

Default

Super Moderator
Platinum Supporting
Joined
Jul 30, 2007
Messages
13,596
Reaction score
3,008
Location
Philly, or thereabouts
Guild Total
11
That's indeed odd that this trebble pot in 10 position would drop the voltage from 7.68V (#6) to 1.32V (#7). Let's see what Steve has to say. The good thing, if this pot would be the source of trouble a replacement would be no problem.
Ralf

How's the sweep of the pots? Does everything measure out ok? As for the voltage drop/volume drop, I would suspect some sort of short to ground or an issue internal to the pot. does it meter out ok? Unfortunately, I don't know what the loss is with a Baxandall tone stack. IIRC, it's more lossy that a Fender, but it can't be this lossey.
 

SFIV1967

Venerated Member
Joined
Nov 20, 2010
Messages
18,442
Reaction score
8,956
Location
Bavaria / Germany
Guild Total
8
Stackpole Electronics, Inc.
A "Stackpole Electronics, Inc" didn't exist in 1970 (jut to be correct...). The pots were most probably in the Electronics division of the Stackpole Carbon Company in St.Marys, PA or in 1970 already under the Stackpole Components Company in Farmville, VA and/or Raleigh, NC. Hard to say as I didn't find any vintage components catalogs from that time. The new SEI from the 90ies has nothing to do with those vintage parts anymore, unfortunatelly.
Ralf
 
Top