Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jack_Hester on June 17, 2015, 08:51:33 am

Title: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on June 17, 2015, 08:51:33 am
I have a Guild Thunderstar combo that has a split chassis.  Viewing from the rear, one chassis is mounted on end, to the left cabinet wall.  This would be the PS, PI, and Power Amp.  The other chassis is mounted across the top, with faceplate to the front. 

This amp had a spring tank that had a broken spring (of two), an open input on any ohm setting, and the Reverb transformer is cooked.  I have not tested the 6GW8 tube, but I did pull it and brought the amp up on the lamp limiter.  Amp sounds fine, though the Tremolo is not very strong.  I can address that later.  My main focus is to repair the Reverb circuit. 

The speaker has it's own enclosure on the right side of the cabinet, the left side of which making for a center wall in the main cabinet.  The spring tank is mounted vertically (output up) facing the Power Amp.  The speaker enclosure has it's own back panel. 

I have not pulled anything other than the spring tank, to take DC resistance readings.  I didn't record the exact output resistance, but it was just a little over 200 ohms. 

Referring to my attached schematic, can anyone tell me where to find information on a '4G' spring tank?  Mercury Magnetics offers a replacement transformer for the numbers on mine: GUILD-THUN-RT ( 007013 9264761).  However, there are no Primary or Secondary impedances listed.  I have one of these ordered, but they say that it is a non-stock item and will have a week to 10 days turn-around.  Plans are to run the specs on theirs, for myself.  Turns and impedance ratios.  The Guild Thunder-Superbird schematic shows the same tank, as well as the 68 ohm resistor in series with the tank input.

The plate resistance of the 6GW8 pentode half is 48K (RCA data sheet).  The cathode for this has a 170 ohm resistor, bypassed with a 10uf cap. 

How would I go about calculating the Primary impedance of the Reverb transformer, to match these specs?  Not knowing the tank input impedance (or DC resistance), I don't know where to start for the Secondary. 

Thanks for any help.  Have a good one. 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: sluckey on June 17, 2015, 10:23:22 am
The tank you probably need is a 4AB3C1F. "F" denotes mounted on end with output jack at top. Good luck finding one. A 4AB3C1B is easy to find and will probably work OK mounted on end.

6GW8s are very expensive. Hope the one you have is good.

I would not bother trying to calculate impedances for the transformer. I also would not use a MM reverb transformer. The cheap Fender reverb OT should work just fine in that circuit with the tank #s I mentioned.

Here's a good pdf about reverb tanks...
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on June 17, 2015, 12:00:45 pm
Quote from: sluckey on June 17, 2015, 10:23:22 am
The tank you probably need is a 4AB3C1F. "F" denotes mounted on end with output jack at top. Good luck finding one. A 4AB3C1B is easy to find and will probably work OK mounted on end.

I've got a 4AB3C1B on hand.  Made by MOD. 

Quote
6GW8s are very expensive. Hope the one you have is good.

I've got some of those, too.  Just haven't tested them.

Quote
I would not bother trying to calculate impedances for the transformer. I also would not use a MM reverb transformer. The cheap Fender reverb OT should work just fine in that circuit with the tank #s I mentioned.

I only calculate (or try to) just so I can document what I do. 

Why not use the MM transformer?  I supposed that I should have posted this before I ordered.  Are they not reliable?  I sent them an email asking the specs of their transformer.  I may call.  Even if it's not a good one, I can use it to determine the ratios or my records (and here). 

Quote
Here's a good pdf about reverb tanks...

I had the same copy, named a bit different.  Good info.  Thanks for responding.  I'll post my progress.  Have a good one.

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on June 17, 2015, 12:41:52 pm
Patrick from MM just emailed the specs on their transformer:

5K ohm Primary
8 ohm Secondary 

What is the function of the 68 ohm resistor in series with the Secondary and the tank input? 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: sluckey on June 17, 2015, 12:58:14 pm
Nothing wrong with the MM transformers other than cost. I misspoke about the Fender transformer. I should have recommended the OT used in the 6G15 reverb unit or the Fender Champ. Sorry.

That 68Ω resistor is probably just to reduce drive current.
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on June 17, 2015, 01:24:17 pm
Here's some data found from a Google search:

http://archive.ampage.org/articles/4/gagd/241821/Re_Reverb_Transformer_Impedance_My_measurements.html (http://archive.ampage.org/articles/4/gagd/241821/Re_Reverb_Transformer_Impedance_My_measurements.html)

Quote
(http://archive.ampage.org/graphics/icons/post_old.gif) 5/22/2003 7:55 PM
Tom Phillips (http://archive.ampage.org/authors/Tom%20Phillips-1.html)
Re: Reverb Transformer Impedance? My measurements
I have taken data on  several of the Fender reverb drive transformers. Following are some of my measurements: 
 
Part number 125A20B as used in the BF combo amps. 
Turns ratio  48.5:1 
Calculated impedance ratio 2,352:1 
Therefore, assuming an 8 ohm reverb tank input transducer, the primary impedance would be 18,818 ohms. 
 
Part number 125A12A as used in the 6G15. 
Turns ratio  30.15:1 
Calculated impedance ratio 909:1 
Therefore, assuming an 8 ohm reverb tank input transducer, the primary impedance would be 7,272 ohms. 
 
Hope I did the math correctly. I don't have an explanation of why my numbers differ significantly from some of the others reported. Comments welcome. 
 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on December 03, 2015, 05:28:10 am
Been several months since I had this amp out.  I put it away, until I had a Reverb transformer on hand.  That came in some time back, but I had already started another repair project.  So, this one had slipped my mind, until I noticed it yesterday, sitting with parts.  Think I'll put it up on the bench today, and see if I can get something started. 

I had already ordered a Mercury Magnetics Reverb transformer, before SLuckey suggested:
Quote
6G15 reverb unit or the Fender Champ
I have a Hammond 1760C that I could try:

http://www.hammondmfg.com/guitarLineOT.htm (http://www.hammondmfg.com/guitarLineOT.htm)

In reply #3, I posted the specs of the MM transformer.  The 1760C is 7K Pri with a multi-tap Sec.  I could do the math and probably find a combination close to that of the MM.  However, the MM came in at some point, so I will use it.  Here's the listing for it:

http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_misc.htm (http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_misc.htm)

Scroll down to Reverb transformers:
GUILD-THUN-RT
 Guild Thunderbird Reverb  # 007013 9264761
The Pre-amp/Reverb/Tremolo chassis is mounted separately from the Power amp chassis, held in by two 4" screws.  This chassis is completely boxed in, so the screws go all the way through, and thread in to the cover.  Makes for easy access.  I don't remember what the insides look like, so I'm hoping this will be a quick replacement. 

I'll have a report on my progress later on.  Maybe some pictures.  Have a good one. 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on December 03, 2015, 04:19:24 pm
Well, success with the MM Reverb transformer.  Tested the 6GW8 and compared it with another that I have.  Both were identical, so I re-used the original.  Installed my 4AB3C1B tank.  Warmed up the amp and gave the Reverb a test.  Good and strong.  So, I'm pleased with this fix. 

Tremolo was not as weak as I originally thought.  I will go through all the tubes and put them on the tester.  It may even be the LDR, but for now I can live with it.

The one big issue now is considerable hum when the Reverb is on.  The footswitch grounds the signal leaving the tank.  This hum dies when the switch is closed, though there is some hum still present.  I'll deal with this when I've resolved the Reverb hum.  The Normal channel is very quiet, with a little hiss when the volume is above 3/4. 

I've posted a schematic of this amp.  I may work on it a bit more, tomorrow, though I have considerable wood to cut.  A very old (maybe 100+ years) pecan tree fell a couple months back.  We had 15 days straight of non-stop rain.  This tree was full grown when I was small.  Two halves.  One fell, and then a couple days later, the other.  Neither fell hard.  They each just laid down.  Never heard them, and they are both right outside the bedroom window, maybe 10 yards away.  One died, right off.  The other was still fully alive, and the leaves came off with the first hard frost.  I'll leave that one, to see if it comes back next Spring.  Not in the way. 

Anyway, chores first.  Then, more on the amp.  Have a good one. 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: Willabe on December 03, 2015, 04:52:21 pm
Sorry to hear that about the pecan tree Jack.

You know pecan wood makes for great slow smoked BBQ.   :icon_biggrin:
Title: Re: Guild Thunderstar (Lead?).....
Post by: 2deaf on December 03, 2015, 05:38:43 pm
The sled in my MOD 4A's have a bushing that is lightly touching the housing when stood on end.  The pegs also touch those bushings when stood on end.  With a pair of fine, long-nose pliers and some patience, you can move the mounting springs to center the sled.  Attached is a suggested placement for the springs.     
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on December 03, 2015, 07:08:57 pm
Quote from: Willabe on December 03, 2015, 04:52:21 pm
Sorry to hear that about the pecan tree Jack.

You know pecan wood makes for great slow smoked BBQ.

My wife and I used to do 18th Century Living History.  So, cooking over a fire is something that I enjoy.  My neighbor wants the dead tree, as he heats his home with wood.  I told him that he could have most of the wood, but I want a goodly pile, for cooking my Brunswick Stew.  I have a 17 gallon pot and use wood, only, to cook my stew.  Won't be suitable this Winter.  But, it will be dry enough for next year. 

Quote from: 2deaf on December 03, 2015, 05:38:43 pm
The sled in my MOD 4A's have a bushing that is lightly touching the housing when stood on end.  The pegs also touch those bushings when stood on end.  With a pair of fine, long-nose pliers and some patience, you can move the mounting springs to center the sled.  Attached is a suggested placement for the springs.   
Thanks for the tip.  I'll give it a look, the next time I'm in the amp. 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on December 04, 2015, 08:38:44 am
Had a thought last night, so this morning I fired the amp up before tending to chores. 

Referring to the schematic, the tank output shows a connection to the RCA jack labeled as RED.  I warmed the amp up, and turned the Reverb volume to 1/2.  Switched it on (footswitch open), and got the loud hum.  I pulled the cable from the RED jack and all Reverb hum disappeared. 

As stated before, I'm using a tank that SLuckey suggested, and one that I had on hand.  4AB3C1B.  Wished that I had saved the old broken tank.  I usually do, but I can't put my hands on it.  Must have put it in the dumpster.  I could have at least check the connectors to see if they were grounded or ungrounded.  The 'C' designation on this one states that it has an insulated input, and a grounded output. 

Anyway, time to get to the outside chores.  If anyone has suggestions on eliminating the hum that seems to be induced by the tank, please comment.  I'll check back in awhile.  Have a good one. 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: sluckey on December 04, 2015, 09:17:11 am
If you totally unplug the footswitch does the reverb work with no hum? Reverb cable to the footswitch must be properly shielded.

Couple other things to try...

Put a 100K resistor directly on the RED phono jack, connected hot to ground.

Short that .1µF cap between the RED jack and pin 1 of the 6GW8.
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on December 04, 2015, 10:36:31 am
Quote from: sluckey on December 04, 2015, 09:17:11 am
If you totally unplug the footswitch does the reverb work with no hum? Reverb cable to the footswitch must be properly shielded.
With footswitch unplugged, hum is always there as long as Reverb volume anywhere above minimum.  Only when unplugged from the tank does the hum disappear.
Quote
Put a 100K resistor directly on the RED phono jack, connected hot to ground.
Meaning the signal path, as in the junction of the RED and the White phono jacks?
Quote
Short that .1µF cap between the RED jack and pin 1 of the 6GW8.
Putting the 100K in parallel with the 33K? 

Just came in for a break.  Will have to drop the chassis out and remove the bottom cover.  There is a disadvantage to having two chassis'.  I will have to remove both, or roll the cab up on end so that the umbilical will reach the power amp / power supply chassis.  I'll probably do that.  Easier.

Back when I started this thread, I had installed a ground jumper between the Power amp chassis (one mounting screw on the PT) and the spring tank.  I just now disconnected this ground to see if it made a difference.  While I was disconnecting, I noticed that some of the hum went away.   When the jumper was removed, same loud hum.  I scraped the tank and reinstalled the jumper.  It reduced the hum, less than when I initially installed this tank.  Bad hum still there, but some reduced.  So, I didn't have the best of grounds, until I scraped the tank. 

Anyway, back to chasing the hum.  More later. 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: 2deaf on December 04, 2015, 03:19:44 pm
Quote from: Jack_Hester on June 17, 2015, 12:41:52 pm
What is the function of the 68 ohm resistor in series with the Secondary and the tank input? 
The 68 ohm resistor functions to make the driver more constant current.  68 ohms is much larger than the 8 ohm nominal impedance of the tank so that when the impedance of the tank changes, the percentage change of the total impedance of the series RL is minimal.  A constant load with changing frequencies means the secondary voltage will be constant and, therefore, the current will be constant.

This is an RL series circuit so the total impedance is not the sum of the two impedances.  This also helps to keep the change in total impedance minimal when frequency changes.  I will work out some numbers when I get home and post them to make what I am saying more clear.


Title: Re: Guild Thunderstar (Lead?).....
Post by: 2deaf on December 04, 2015, 07:18:07 pm
Attached is a table showing what happens with a 68 ohm resistor in series with a 4AB3C1B tank driven by a 5K : 8 transformer.

From 500Hz to 4000Hz the tank impedance increases by a factor of eight, but the current through the tank input coil barely changes.  The primary and secondary impedances only have about a ten percent range so that the voltages don't change very much with frequency.

This works so well with an eight ohm tank that it supports the contention that a 4Axxxxx is an appropriate choice.  You also wouldn't want to stray very far from the 5K : 8 transformer, so I think you did the right thing when you got the MM.
Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on December 05, 2015, 05:41:48 am
That's good stuff.  I always loved math, but was never any good with it.  I made good grades in math, way, way, back.  I don't remember how I did, but it was all mine.  And then, it just sorta leaves my head. 

Would it be much trouble to post the math that got you to this point?  Just for reference, though I'll study it a bit.  Nothing sticks up top, these days.  But if it's in print, I can at least come back to it. 

This particular tank has the input insulated, and the output grounded.  Why would you not want the input to be grounded, as well?  The schematic shows the tank casing grounded to the same point of ground as the Reverb transformer.  Just so I don't have to refer to the spec sheet, I'll post the character designations:

1ST       TANK TYPE (Tank Length and Number of Transmission Springs)
2ND      INPUT IMPEDANCE (Measured at 1kHz)
3RD      OUTPUT IMPEDANCE (Measured at 1kHz)
4TH      DECAY TIME (Short, Medium or Long)
5TH      CONNECTORS (Insulated / Non-Insulated Configuration)
6TH      LOCKING DEVICES
7TH      MOUNTING PLANE (Transducer Magnet Adjustment)

Just to satisfy my curiosity, I will trace the wiring on the transformer, and confirm it with the drawing. 

When the sun comes up, and warms the shop a bit, I'll get back into the amp.  More to come.  Have a good one. 

Jack
Title: Re: Guild Thunderstar (Lead?).....
Post by: Willabe on December 05, 2015, 08:17:37 am
Quote from: Jack_Hester on December 05, 2015, 05:41:48 am
This particular tank has the input insulated, and the output grounded.  Why would you not want the input to be grounded, as well? 

Ground loop buzz.
Title: Re: Guild Thunderstar (Lead?).....
Post by: 2deaf on December 05, 2015, 01:58:38 pm
Quote from: Jack_Hester on December 05, 2015, 05:41:48 am
. . . to post the math that got you to this point?
The math is very simple, you just need to know the pertinent equations.  I have attached those equations.

R = Resistance of the resistor in ohms.

XL = Inductive reactance of the input coil in ohms.

f = frequency in Hz.

L = Inductance of the coil in H.

ZT = Total impedance of the resistor and coil in series.

VT = Total voltage across the resistor and coil in series.

IT = Total current through the series RL circuit.

IR = Current through the resistor.

IL = Current through the coil.

You're given XL at one frequency (8 ohms @ 1KHz) and L is constant, so you can calculate XL at other frequencies .  Whatever you multiply f by, you multiply XL by.

You're given R (68 ohms) and you can calculate XL, so you can calculate the total impedance of the series RL circuit at any frequency using a calculator that will do square roots or a slide rule.

You're given the total voltage across the series RL (2V rms) and you can calculate the total impedance, so you can use Ohm's Law to calculate the total current through the series RL circuit.  The current is the same everywhere in the circuit, so the current through the input coil is the same as the total current.

You're given the impedance ratio of the transformer (5000 : 8  or  625 : 1), so you can calculate the primary impedance by multiplying the secondary impedance by 625.

Title: Re: Guild Thunderstar (Lead?).....
Post by: Jack_Hester on December 05, 2015, 04:20:36 pm
I didn't get back to this one today.  Took a break and opened the Sano Supersonic:

http://el34world.com/Forum/index.php?topic=18563.msg202513#msg202513 (http://el34world.com/Forum/index.php?topic=18563.msg202513#msg202513)

I'll start back again, maybe on Monday.  Have a good one. 

Jack