Bias issues with '68 Guild Thunderstar Bass..........

[Jack_Hester]

This afternoon, after Church, I put an amp head on the bench, mainly to inspect and clean.  As it looked so good inside, I decided to connect a speaker (schematic called for an 8 ohm), install the bias tester sockets, and see what things looked like.

Right off, I noticed no bias current on one tube and way off scale on the other.  As this amp has a bias pot for each power tube, I rotated them from one end to the other.  The tube with no current never changed.  So, I centered that pot and tried the other.  It made a bit of difference, but the current kept climbing.  The chassis was upside down at the time, and so I took a quick look underneath.  That's when I saw a power tube red-plating.  Before seeing that, I did manage to take some voltage readings.  I shut it down quickly. 

I had 499vDC on the plate of the tube with no bias current flow.  484vDC on the one that was off scale.  With each bias pot centered, I had -34 to -35vDC measured at pin 5 of each tube.  I did not manage to get screen voltage readings for either.

Anyway, I haven't tested the 6L6GC's, but I'm satisfied that I damaged one. 

As stated above, I only planned to do some cleanup and play with the amp.  I was curious as to how well the 'Pre-amp output/Power amp input' would work with a pedal.  A really nice, well built amp.  This one has 4 tubes, but there is a schematic of a 5 tube model.  I have the 4 tube schematic on the bench.  Maybe I'll get a chance to troubleshoot a bit this week. 

Please take a look at the schematic and make any suggestions or observations.  I'll post any findings that I make, when I'm able.  Thanks.  Have a good one. 

Jack

[HotBluePlates]

... I saw a power tube red-plating.  ... Anyway, I haven't tested the 6L6GC's, but I'm satisfied that I damaged one.

I know that's the conventional wisdom; however, I have a 6V6 which redplated to an extreme that there's a hole melted in the plate (happened before I acquired it, in a lot of tubes). It measures, plays and sounds just fine... I'll try to dig it out and take a picture.

Please take a look at the schematic and make any suggestions or observations.

The schematic indicates it has a bias test jack, and cathode resistors for each tube. Maybe use that instead of your bias probe, verify and use the on-board system.

If one of those cathode resistors is open or has a bad connection, the attached tube will pass no current.

Yank the output tubes. Eliminate leaky coupling caps as a possible problem by unhooking one leg of each of those 0.27uF caps (that's an odd value...). If the mu from G1-G2 is 10, then a bias voltage 1/10th of your screen voltage will just about shut off tube current. For a 6L6, mu G1-G2 is 8, so you'd like your maximum bias voltage to approach G2 volts/8. If you get near that (schematic shows maximum bias at -44v) and confirm good connection for the cathode resistors, try re-installing the 6L6's while still keeping the phase inverter coupling caps unhooked. See if you can get reasonable idle currents.

I think you'll find a bad resistor or connection accounting for lack of tube current in the one tube (the external bias probe setup adds another layer of possible failure). Redplating will either boil down to lack of sufficient bias voltage (maybe failing bias caps) or leaking phase inverter coupling caps.

[Jack_Hester]

HBP -

Well, you nailed it on the cathode resistor.  The tube that had no current flow has an open 6.8 ohm resistor.  The other tube cathode resistor measures 11 ohms.  I'll replace them both.  I checked them a few minutes ago. 

Also, the amp has a very noticeable hum with the volume all the way down.  I'll replace the power supply caps and the two bias caps, and go from there.  I can drop a couple .1uf caps in place of those .27uf coupling caps.  I have some of those on hand.  Hopefully, the other parts will be in before the weekend. 

I'll test all the tubes, and I hope that I didn't damage the one that red-plated.  But, I have a replacement, so all is not lost.    I'll use it if it is good. 

I'll post my findings after the component changes.  Thanks ever so much for the help. 

Jack

[HotBluePlates]

Also, the amp has a very noticeable hum with the volume all the way down.  I'll replace the power supply caps and the two bias caps, and go from there.

If the output tubes are in, pull the phase inverter (if you haven't already unsoldered one leg of the coupling caps). If you still have hum (which could only be at the output tubes at that point), the culprit is very likely hum in the bias supply due to failed bias filters.

Now you could go in and replace a lot of caps. But it might be more enlightening to try to pin down just the bad parts (and cheaper, too). Of course, if you have hum with no output tubes in, you really should ask how could that get to the speaker...

[Jack_Hester]

If the output tubes are in, pull the phase inverter (if you haven't already unsoldered one leg of the coupling caps). If you still have hum (which could only be at the output tubes at that point), the culprit is very likely hum in the bias supply due to failed bias filters.

Now you could go in and replace a lot of caps. But it might be more enlightening to try to pin down just the bad parts (and cheaper, too). Of course, if you have hum with no output tubes in, you really should ask how could that get to the speaker...

True.  And, I don't have a hankering to replace a lot parts.  Truth is, I make my 'repair' orders as a worse-case, but I like to do just a bit at a time.
 
And, I thought of something else, right after I shut the computer down for the night.  When the amp is in Standby, it is super quiet.  Just a ever so slight hum that might be corrected with the Hum Balance pot, once the louder hum issues have been resolved.  As this Standby grounds the signal from the Pre-amp, I'm thinking everything downsteam will may be ok after the cathode resistors on V3 and V4 have been replaced.  I did not order any 30uf caps to replace the ones on V1.  I figured to use 25uf there, if I find they need replacing.  Got plenty of those on hand. 

Anyway, I'll test the power tubes, replace the cathode resistors, take some voltage readings, and see if the Bias can be dialed in.  Then, work my way back to the hum (assuming it's still there).  Really looking forward to experimenting with this amp.  It's been put away for a couple years, and I only got it out because something reminded me of it the week before.  So, it went to Church yesterday for show-and-tell.  Even the non-electronics types there enjoy these old jewels. 
 
Parts may be here as early as Wednesday.  But, more than likely Thursday.  Meanwhile, still no frost, so the grass is still growing.  Keeps me occupied, until parts show up.  More to come.  Have a good one.
 
Jack

[sluckey]

I would replace those 6.8Ω resistors with 1Ω resistors. That would make my life simpler for biasing.

I just did some simple math with the voltage readings provided by the schematic and I get some fantastic numbers! I would not trust the schematic voltages. I'm very curious to see what your real measured numbers are.

Schematic shows B+1 as 470V. I'm assuming that to be plate voltage so these numbers may be a bit off depending on the actual plate voltage. There is .414v across the 6.8Ω cathode resistors. That means there is a whopping 86.25ma flowing thru each tube! Idle plate dissipation (disregarding screen dissipation) would be 470 X .08625 = 40.5W! That seems very hot for a tube that's is specified for 30W max plate dissipation.

[Jack_Hester]

I would replace those 6.8Ω resistors with 1Ω resistors. That would make my life simpler for biasing.

I just did some simple math with the voltage readings provided by the schematic and I get some fantastic numbers! I would not trust the schematic voltages. I'm very curious to see what your real measured numbers are.

Schematic shows B+1 as 470V. I'm assuming that to be plate voltage so these numbers may be a bit off depending on the actual plate voltage. There is .414v across the 6.8Ω cathode resistors. That means there is a whopping 86.25ma flowing thru each tube! Idle plate dissipation (disregarding screen dissipation) would be 470 X .08625 = 40.5W! That seems very hot for a tube that's is specified for 30W max plate dissipation.

I've got 1 ohm resistors in 1, 3, 5, and 10 watts.  I wonder why they chose 6.8 ohms to measure bias across. 

I didn't write down the voltages, as this was supposed to be a quick test.  But, the B+1 was greater than 500vDC.  I'll solder in a couple 1 ohm resistors this evening, install a couple known good 6L6GC's, have my bias tester in place, and my meter ready to take some quick B+ readings.  If I can, I'll dial the bias in on both tubes and take a complete set of readings on all 4 tubes. 

I didn't really want to trust the bias voltage readings, which is why I use the socket adapters.  I just recently found one (adapter) that allows for measuring plate voltage as well as the cathode bias current (mV's, that is).  It was a bit pricey, but already I've found a need for such, when I couldn't take plate voltage readings to accurately figure my bias current.  Here it is:

http://www.ebay.com/itm/291197166760?_trksid=p2060778.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Thanks for giving me more to try.  I'll post the results this evening.  Have a good one.

Jack

[sluckey]

I've got 1 ohm resistors in 1, 3, 5, and 10 watts.  I wonder why they chose 6.8 ohms to measure bias across.

Probably because when that amp was built there were no affordable digital meters that were capable of accurately measuring 40mV. So a larger resistor puts the voltage nearer to .5V which was easier for the meters of the day to measure. But why specifically 6.8Ω? Maybe the mfgr had a devil's deal with Ohmite? Who knows. 10Ω would do the same thing and not require a slide rule.   

[Jack_Hester]

I was just discussing this post with a buddy, who is in the process of building a really nice SLO Clone.  Mostly the discussion was about how little troubleshooting I've had to do, as the answers keep popping up here.  There are so many projects in front of this, but I just can't leave a problem when one rears it's head.  I've not fixed anything, yet.  But, I'm satisfied that the fix(s) are soon to come, with the information that I have at hand. 
 
So, I'm really grateful for the quick reponses.  I won't mind it one bit, if I wind up with unnecessary repair parts.  They'll go into inventory with the rest.  I'll post a picture of the amp, when it's cleaned up, and back together.  Thanks a bunch. 
 
Jack

[HotBluePlates]

... There is .414v across the 6.8Ω cathode resistors. That means there is a whopping ...

I got ~61mA, for a bit under 29w plate dissipation (a little less due to screen current). Sounds like running a true 6L6GC in class A.

But I agree that a 10Ω resistor would make more sense.

[Jack_Hester]

Tested both 6L6GC's, and even though I don't have an analyzer, my B&K 707 showed each to be strong tubes. 

Decided to go with the 1R-3W resistors on the cathode of each power tube.  Clipped the old ones and installed the new.  Installed the bias adapters and tubes.  Powered the amp and watched the bias readings.  Both were in the 60mA range for maybe 2 or 3 minutes.  Then, both began climbing.  A quick look at the tubes and both power tubes were beginning to red-plate.  By that time, the bias on each was right at 100mA and showing no signs of slowing down.  So, off with the power and power tubes removed.  I left the two other tubes in place. 

I centered both bias pots and put power to the amp.  Here are the initial voltage readings:

PT Primary: 123vAC
PT Secondary: 198vAC
B+1: 533vDC
B+2: 490vDC
B+3: 374vDC
B+4: 323vDC
DC to Bias Pots: -46vDC
Bias Pot Wipers: -37vDC/-38vDC

I know the overall voltages should drop some, with the power tubes in place, and a bit more load is on the PT.  I didn't want to leave the power on, so I missed recording plate voltage readings. 

That's all I'll be able to do tonight, but I'll get in an hour or so tomorrow evening.  Got a notice that my parts will be delivered Thursday, so I'll have those on hand if needed.  More to come, then.  Have a good one. 

Jack

[HotBluePlates]

... Powered the amp and watched the bias readings.  Both were in the 60mA range for maybe 2 or 3 minutes.  Then, both began climbing.  A quick look at the tubes and both power tubes were beginning to red-plate.  ...

With phase inverter coupling caps unsoldered?

Otherwise, it sounds like something fails after it's hot. It would be nice if your meter had been monitoring bias voltage at the time the current started climbing (of course, it's always nicer to have several meters watching different things in these cases).

[sluckey]

I got ~61mA, for a bit under 29w plate dissipation

Oops! 

That's what I get tonight. I think my AM calculator needs calibrating.   

[sluckey]

Both were in the 60mA range for maybe 2 or 3 minutes.  Then, both began climbing.

That really sounds like leaky coupling caps. But if they were disconnected then the bias voltage must be decreasing as the amp warms up. Probably bias filter cap(s) in that case.

[Jack_Hester]

With phase inverter coupling caps unsoldered?

Otherwise, it sounds like something fails after it's hot. It would be nice if your meter had been monitoring bias voltage at the time the current started climbing (of course, it's always nicer to have several meters watching different things in these cases).

Negative on the coupling caps.  They are soldered very close, and will be sacrificed when removed.  Not a problem.  I can install a couple .1uf caps.  The .27's will come out tomorrow.  I have another meter that I can use to monitor the bias supply. 

Both were in the 60mA range for maybe 2 or 3 minutes.  Then, both began climbing.

That really sounds like leaky coupling caps. But if they were disconnected then the bias voltage must be decreasing as the amp warms up. Probably bias filter cap(s) in that case.

I'll verify the voltage tomorrow.  I know that I have a 100uf-150v cap, but not sure about a 50uf.  I've got a couple of each coming Thursday.  I just took a look at the ones installed.  The 100uf-150v is there.  However, there is a 500uf-50v where the 50uf should be.  It's easy to verify.  Both will be easy to replace. 

Anyway, I'll clip the coupling caps from the PI, and report on the status when powered up. 

Thanks to both of you for your input.  Have a good one. 

Jack

[PRR]

> .414v across the 6.8 cathode resistors. That means there is a whopping 86.25ma

On my abacus, 0.414V in 6.8 Ohms is 0.06088A.

EDIT... I just saw HBP saying numbers like this. Back to my blather...

> not require a slide rule.

You should have one handy. Slide-rule gives a number like 61 (decimal point is up to the user).

I'm suspecting you saw 6.8 but your finger hit 4.8.

That's cathode current. Plate current will be less but in 6L6 the difference is small.

Taking 55mA-61mA, Plate diss looks like 26W to 28.61W. Nominally inside the 6L6GC's "30W" number.

> some fantastic numbers!

0.414 is pretty fantastic for that date. You *could* read a mirror-scale 5" meter to three digits, but few were calibrated that close. And obviously you tweak the nominal -36V grid voltage to *get* roughly 0.414 (or 0.4) at each 6.8 Ohm resistor.

The bias adjustment is not generous and may have to be extended to get some new-made 6L6 into a good zone.

I do not see any reason this amp would not play just-fine under the "70% rule". Idle at 70% of rated Pdiss. 70% of "30W" is 21 Watts. 21 Watts at 470 Volts is 0.044 Amps or 44mA. In 6.8 Ohms that reads as 0.304 volts (write "0.3V" on the jack and be done with it). In 1 ohms it would be 44mV of course.

You could try running 60mA, but I do not think sound will "improve" until you get well over 100mA-150mA and then the Pdiss would kill the tubes quick. Also I do not think the "improvement" will matter for something sold as "THUNDERstar".... obviously this is not about the soft off-idle sound but the THUNDER. Which will be about the same for "any" bias that doesn't suck bad.

And in my limited experience, new-made and some old-age 6L6Gc are not long-term reliable at 30 Watts Pdiss, though the nice low (91K) gridleaks do help with this.

Interesting that V4 is fed from a "wrong point" on the V2B cathodyne. 15K on top and 1K+14K on bottom. You would think they would take the 15K point on bottom for "balanced drive". (Maybe they did but mis-drew it.) I do not think this would be bad for sound; it might even flavor the thunder.

> those 0.27uF caps (that's an odd value...).

Might be Williamson influence? I can see the whole thing built from a late Williamson Kit. Two little tubes, two big tubes. Even a hum-balance. Modded for early crystal rectifer. Might have been using-up some supplier's stock of chassis and OTs and 0.27 caps.

[Jack_Hester]

The bias adjustment is not generous and may have to be extended to get some new-made 6L6 into a good zone.

Interesting that V4 is fed from a "wrong point" on the V2B cathodyne. 15K on top and 1K+14K on bottom. You would think they would take the 15K point on bottom for "balanced drive". (Maybe they did but mis-drew it.) I do not think this would be bad for sound; it might even flavor the thunder.

Been thinking on the possible need to extend the range of the bias adjustment(s).  I'll take a reading of the AC before and after the 5R - 5W, to give me some idea of how much voltage I have.  And, if I have enough to rescale the range if needed.  We'll see what the final overall result of the fixes leave it, and go from there.
 
Glad you pointed out the connection of V4.  I was some tired when I traced the coupling caps, but I don't believe that V4 is connected as shown in the schematic.  I was paying more attention to how I would disconnect the .27uf cap, but couldn't make the connections match the drawing.  I'll try to remember to confirm that this evening. 
 
Memory does not serve me well here, but I measured the voltage at pin 5 of each power tube, at some point.  I don't believe that I had higher than maybe a couple volts less than that at the wipers of each bias pot.  I'll have to document that this evening, as well. 
 
Anyway, I appreciate you taking a look.  All suggestions are a great help.  Have a good one. 
 
Jack

[Jack_Hester]

Thinking on a statement that PRR made, concerning the possibility of extending the bias range, I made a quick drawing at lunch of the Bias Supply in this amp.  I like having it to view, without the remaining amp. 

Assuming that all fixes are in place for the amp, and that the tubes still need this extended range, I'm thinking that I could temporary in a 25K pot, in place of the 2.7K resistor that ties the bias pots to ground.  Of course, not knowing the bias supply AC level (haven't been home to measure it yet), I don't know how much range I can have.  But with a pot here, it can become a bias level adjustment, with the 2 existing pots used to balance the tubes.  The wiper would be tied to the ground leg of the pot. 

Anyway, just some thoughts until I can do some more troubleshooting this evening. 

Jack

Note: See Reply #23 for original Bias Supply and my proposed mod - 29Oct14

[2deaf]

From your numbers, you would have 76V across your bias winding.  4K to 5K from the pots to ground will probably get you what you want (4.7K should yield a range from 40V to 53V).  If you want to vary the resistance from the pots to ground, it should be a pot and a resistor in series so that you cannot turn the bias to zero.

[2deaf]

those 0.27uF caps (that's an odd value...).

They're not that odd to me because I used to use them for cathode cap.'s a long time ago.  They were in a baggie full of assorted cap.'s that I bought for approximately the cost of the baggie right around 1976.  I still have four never used .027uf, 250V, 2.5% "F-Dyne" cap.'s from the same baggie.  They probably had a really big baggie full of .27's.

How about that 5 ohm resistor with 11 ma going across it that is rated at 5W?  Looks like baggie syndrome to me.

[Jack_Hester]

Well, things are looking up.  But, I'll get to that in a minute.

Took some quick voltage readings with V3 and V4 removed.  Left V1 and V2 installed.

V4 -
Pin 3 - 528vDC
Pin 4 - 485vDC
Pin 5 - -38vDC (.27uf coupling cap still connected - pot centered)

V3 -
Pin 3 - 528vDC
Pin 4 - 485vDC
Pin 5 - -38vDC (.27uf coupling cap still connected - pot centered)

Filament - 7.4vAC (this dropped to 7vAC, when I had power tubes in)
Line In - 123vAC
Secondary Out - 197vAC

Had to try them again.  The problem tubes from last night.  Let me note here that these are RCA branded, but in bold print 'Made in Russia'.  They appear to be very modern, though I don't know how new. 

Anyway, I'm monitoring plate voltage on each, as well as screen voltage.  Power on.  Adjusted bias to around 60mA on each and watched.  Did not write the exact voltages down, but they were around 485vDC (screen) and just shy of 490vDC (plate).  I forget what the grid voltages were, but they were steady, a negative voltage, maybe -50vDC more or less.  No moving.  Just steady.    Within minutes, bias current started climbing, and tubes turned cherry.  Power off.  All voltages remained constant.  Steady.

So, I'm wanting to try more tubes.  I go digging.  Come out with a GE 6L6GC, and a Sylvania 6L6GC.  Also, a couple Sovtek 5881WXT's and a couple 6P3S-E's. 

Installed the first two.  Though they are way mismatched, V3 (GE) has all kinds of bias range.  The best that I can do with V4 (Syl) is 46mA.  So, I dialed V3 in to match.  Plate voltage is right at 494vDC on each.  -39vDC on V3 pin 5, and -44vDC on V4 pin 5.  And, all the big hum is gone.  I tweaked the hum balance pot, and you have to put your ear to the speaker to hear any. 

So, decided to try the 6P3S-E's, knowing that they will probably be like 6L6G's.  The best that I bias them was 40mA, with plate voltages being 493vDC.  Did not measure the bias voltage at pin 5.  I left them on, at idle, for at least 15 - 20 minutes.  Bias was steady, and amp was quiet. 

So, the two old stock tubes are back in and biased as before.  They have been in now for a good 30 minutes.  Steady. 

I don't have specs on the Sovtek 5881WXT's, as far as plate dissipation, so I'll not try them.  They are new. 

Anyway, looks like the original Russian RCA's were the source of the problems, possibly why the one cathode resistor was open.  It is very small.  Possibly a 1/4 watt.  Just that small.  The other 6.8 ohm is at least 1 watt, maybe larger.  And, appears to be original to the amp. 

Tomorrow, I'll take a complete set of voltage readings, with the old stock tubes in place.  Unless I find some info on the 5881WXT's, I'll not try them.  I'll post tomorrow's findings and try the amp out with the guitar.  Please comment, if you have any thoughts as to other things to try.  Have a good one.

Jack

[Steve_P]

Jack, is this a Thunderstar or a Thunderbass? Those B+ and bias readings are typical for a Thunderbass that ran 2x8417's @ 100 watts. 8417's can be had at Lost Art Vintage, if you don't have a supplier.
Guild had a lot of undocumented running changes, fwiw. Also, Aaron Newman was the designer for Guild's Sixties amp lines, so you are seeing the Lafayette hifi pedigree in that amp design.

[Jack_Hester]

Jack, is this a Thunderstar or a Thunderbass? Those B+ and bias readings are typical for a Thunderbass that ran 2x8417's @ 100 watts. 8417's can be had at Lost Art Vintage, if you don't have a supplier.
Guild had a lot of undocumented running changes, fwiw. Also, Aaron Newman was the designer for Guild's Sixties amp lines, so you are seeing the Lafayette hifi pedigree in that amp design.

It's a Thunderstar, and marked for 6L6GC's.  But now, you've given me another tube to be on the lookout for, when I'm back to sorting through my old tubes.  That's my rainy day, cold weather chore. 
 
Really good information.  Thanks for posting.
 
Jack

[Jack_Hester]

From your numbers, you would have 76V across your bias winding.  4K to 5K from the pots to ground will probably get you what you want (4.7K should yield a range from 40V to 53V).  If you want to vary the resistance from the pots to ground, it should be a pot and a resistor in series so that you cannot turn the bias to zero.

Been thinking more along these lines.  I added a pot in series with a resistor, in place of a singe resistor, on my Oliver PA-100X.  That one had a 47K that I replaced with a 25K pot and a 24K resistor in series.  I think that I have a 10K pot that I can series with say a 1.5K resistor, and solder the combo in place of the 2.7K.  And, I think that there is already a hole on the surface of the chassis, near the OT, that is large enough for the pot.  I'll explore these options this evening, as well. 
 
I'm hoping to find some plate dissipation specs on my 5881WXT's, so that I can give them a try.  I really like the idea of being able to bias a variety of tubes.  Who knows.  I may get back to my original intent of cleaning the cabinet, and trying out a pedal in the 'Pre-amp Out/Pwr Amp In' loop.  I'd almost forgot how this all came about.
 
Jack

[Jack_Hester]

Think I may have found the info I need on my Sovtek 5881WXT's:

http://www.jayskyler-guitarlessons-sanfrancisco.com/guitar-gear-guide/sovtek-5881-wxt-6l6-wgc-tube-guide-2.html

Jack

[Jack_Hester]

Well, I'm really happy with the mod to the bias supply.  Really good range of adjustment.  Gave it another trial with the old stock tubes.  Then, I swapped them out for some coin-base 6P3S-E's.  The plate voltage is running around 507vDC, and I biased these tubes to just a bit over 31mA.  Been holding steady for the past half hour, amp nice and quiet, with a little background hum.  I attribute that to the fact that the chassis is belly-up.  I'll clean the cab tomorrow afternoon, and slip the chassis back in.  Think I'll leave these tubes in while I'm testing the amp.  They may stay.

I'll have a complete set of readings and a voltage chart, before closing it up.  More tomorrow.  Have a good one. 

Jack

[Jack_Hester]

Got in a little late to do much with the amp.  I did take the time to do one more mod.  Mounted a solder terminal strip to one of the studs, and added a thermistor in series with one PT primary lead and the power switch, knowing that it would drop the line voltage a bit, as well.  Really nice soft start.  Before, the plate voltage would sail up to 538vDC, before dropping back to a little better than 500vDC.  Now, it comes up slowly to 490vDC, and tapers back to a steady 485vDC.  6P3S-E's are biased to 33mA, and holding. 

Serious rain coming in tonight, and all day tomorrow.  So, I'll get my readings in the morning, and have it all cleaned up and back together by middle of the morning. 

Jack

[Jack_Hester]

Let the amp warmup for a good half hour and took some readings.  As soon as the cabinet is ready, and the chassis is installed, I'll snap a picture. 

Jack

[Jack_Hester]

Well, it's all cleaned up and chassis re-installed in the cab.  Even quieter in the cab, as there is a sheet of copper stapled to the cab bottom, right under the pre-amp.  The chassis is spacious, and the pre-amp is away from the PI/Pwr amp. 

My little Supro guitar sounds really nice through it.  Got out my Danelectro Cool Cat pedal and connected it to the "Pre-amp Out/Pwr Amp In".  Works really nice. 

I'm going to let the amp cook for a long while, keeping an eye on it, though I'm sure it's just fine.  Bias still holding steady at 32mA, both tubes. 

Here's a couple pictures of the finished amp:





Thanks for the input and troubleshooting suggestions.  Have a good one. 

Jack

[eleventeen]

Looks very homebrewish, does it not? And nothing close to present-day "standards" for boutique amps.


No criticism intended, just sayin'.


Nobody cared about this stuff back then!

[Jack_Hester]

Very simple appearance.  Very much the way you described it.  But very high-end, in construction, meaning well thought out.  As quiet as the chassis was on the bench, bottom up, the subtle hum all but disappeared when installed in the cabinet.  My new favorite amp.  At least until the next one crosses the bench. 

I carry these amps to work or to Church, once they are functional.  When you say that nobody cared about this stuff back then, most today don't even know they exist.  The guys that I show them to are musicians who have be active since the 60's and 70's.  The younger ones are just in awe.  So far, they are amazed that such fine equipment existed outside the Fender/Marshall/etc. world. 

Jack

[PRR]

The significant negative grid voltages on V1 are wrong. Part-Volt neg may be right, but not 4 Volts negative.

The other pins are right, so I suspect a mis-reading.

[Jack_Hester]

The significant negative grid voltages on V1 are wrong. Part-Volt neg may be right, but not 4 Volts negative.

The other pins are right, so I suspect a mis-reading.

That may very well be the case.  More than likely -0.4vDC.  I couldn't be more pleased with it.  The Hum Balance really cleans up what would have been a low level hum, without it. 

At almost full volume (no numbers on the faceplate, but maybe 9 of 10), there is a hiss.  I attribute that to high gain.  At about 3/4 volume, it starts to distort a bit.  Not bad, but not clean.  In the right hands, it would probably sound real good.