67 Pro Verb - Sound Check

[p2pAmps]

Here is a 1967 Pro Reverb in BAD need of a resto.  I like how these 20 dollar bills photo bombed my picture !




http://www.p2pamps.com/1967-pro-reverb/

[eleventeen]

I had a BF Pro Rev (converted to a Twin, I bot it that way in the 70's) as my main gig amp for 20+ years. Little bit of wear on that one. Never failed me.

[sluckey]

Us po folks just use high density refrigeration foam tape under the dog house.

[Ed_Chambley]

I didn't look that bad to begin with.  Not restored, but a cap job and plate resistors could have sent that thing back on on the road.  Course it would not look as good.  I am sure this pro owner will be happy.


I have 2 personal amps I am thinking on restoring.  The 62 Bassman, I finally found some original Oxblood grill cloth and a real faceplate and rear plate that was still wrapped in the old Kraft paper how they were delivered to fender.


I really don't want to use the original eyelet board, but I got the news from an appraisal.  Now as is it is valued at $4300 with original cabinet.  If I use G10 for the board it goes down so I am using the eyelets.  It will be in a 9 of 10 condition then and be at $4600.  That is so strange to me.


OTOH, I am getting ready to restore a Super Reverb, 1971 I just got and I am going to use whatever I want cause I am shaving it down to a 2, 10" for someone to use as a club amp.

[p2pAmps]

I didn't look that bad to begin with.  Not restored, but a cap job and plate resistors could have sent that thing back on on the road.  Course it would not look as good.  I am sure this pro owner will be happy.


I have 2 personal amps I am thinking on restoring.  The 62 Bassman, I finally found some original Oxblood grill cloth and a real faceplate and rear plate that was still wrapped in the old Kraft paper how they were delivered to fender.


I really don't want to use the original eyelet board, but I got the news from an appraisal.  Now as is it is valued at $4300 with original cabinet.  If I use G10 for the board it goes down so I am using the eyelets.  It will be in a 9 of 10 condition then and be at $4600.  That is so strange to me.


OTOH, I am getting ready to restore a Super Reverb, 1971 I just got and I am going to use whatever I want cause I am shaving it down to a 2, 10" for someone to use as a club amp.

On the face it didn't look that bad but believe me it would be in my shop a lot if I just didn't go through it like I am.  I found a dozen things right off the bat.  Owner said to gut it so I did.  It's faster for me to do it that way and I can see all the issues once I do that.  I think it was the better choice considering what we started with honestly.

[Ed_Chambley]

Yep, looks can be deceiving!  It is a 67 Pro, so very worth going through.

[shooter]

Um;

[p2pAmps]

Um;

Little silver box is full of Step Bits LOL

[p2pAmps]

[p2pAmps]

Guys,

I would like some input of this issue.

The PT is very suspect in this Pro.  I know its been hot from all the drips. 

I finished it up today, brought it up on the variac.  Max 370vDC on the plates at 120vAC on the Variac.  Also, I am running a 27K resistor on the bias pot and measuring -112mV for bias, and can't get it lower.  I stuck a 10K on there but still no joy as it bought it down to around -90mV.  I didn't bother with the OT shunt yet as I like getting it in the ballpark first.

So...  Looking for things to check.  I feel like the PT should be around 435vDC for its age as that is what I commonly see. 

I did swap out the 6l6s too just because.  I have some known good ones I use in the shop for these before I commit to the customers tubes...

Thanks in advance
Mike

[eleventeen]

What voltage do you measure at the junction of the 220Ks before the 6L6 tubes? In other words, what actual voltage does your bias supply produce?


Typical is about -45 volts.


Sounds to me like you have something hinky in the bias supply, you're not producing enough bias (eg; "shutoff") volts and the 6L6 tubes are overcurrenting. Bias pot could be shorted or have some dirt in it.


EVEN IF the bias tap is somehow not right (and assuming this does not indicate a more substantial transformer failure) you can still create perfectly proper negative bias using one leg of the HV supply. You use the "same" backwards diode configuration, you just have to fiddle with the voltage divider resistors to whack the negative volts produced. Not difficult.


OR.....per your usual "roadworthy" philosophy, you could just inform the customer that the tranny is on the edge and you reco replacement.


Trannies can and do get hot and work for 50 years. I have seen really ugly drippy ones work fine.

[p2pAmps]

What voltage do you measure at the junction of the 220Ks before the 6L6 tubes? In other words, what actual voltage does your bias supply produce?

-91v

Typical is about -45 volts.


Sounds to me like you have something hinky in the bias supply, you're not producing enough bias (eg; "shutoff") volts and the 6L6 tubes are overcurrenting. Bias pot could be shorted or have some dirt in it.

CHANGED THE BIAS POT FOR A BRAND NEW


EVEN IF the bias tap is somehow not right (and assuming this does not indicate a more substantial transformer failure) you can still create perfectly proper negative bias using one leg of the HV supply. You use the "same" backwards diode configuration, you just have to fiddle with the voltage divider resistors to whack the negative volts produced. Not difficult.


OR.....per your usual "roadworthy" philosophy, you could just inform the customer that the tranny is on the edge and you reco replacement.


Trannies can and do get hot and work for 50 years. I have seen really ugly drippy ones work fine.

I hate to change a working vintage tranny and I am not convinced it is a problem yet?  The plate voltage is low however.
The bias supply is built new and checks out.
this has me scratching my head a little

[HotBluePlates]

No tubes in the amp, right?

Do the a.c. voltages of just the transformer look right? About 340vac per leg of the high voltage, and about 60vac for the bias tap?

Can you try unhooking one or more windings from the rest of the amp circuit to see of the PT "un-loads" at that point?

Just some ideas to sort out whether it's a circuit-issue or a transformer-issue.

[p2pAmps]

No tubes in the amp, right?

Do the a.c. voltages of just the transformer look right? About 340vac per leg of the high voltage, and about 60vac for the bias tap?

Can you try unhooking one or more windings from the rest of the amp circuit to see of the PT "un-loads" at that point?

Just some ideas to sort out whether it's a circuit-issue or a transformer-issue.

Tubes in amp

[HotBluePlates]

Were the millivolt readings you took idle current across 1Ω resistors? I was thinking you had only millivolts of bias voltage available. If that was idle current, then sagged supply voltage sounds typical for the amp...

I'd start with at least the output tubes not in the amp, though preamp tubes won't be much of a load. Eval the PT voltages again in that scenario.

[p2pAmps]

Were the millivolt readings you took idle current across 1Ω resistors? I was thinking you had only millivolts of bias voltage available.

I'd start with at least the output tubes not in the amp, though preamp tubes won't be much of a load. Eval the PT voltages again in that scenario.

I was grabbing voltage off the 220K where the bias pot ties in... 

[HotBluePlates]

Gotcha.

Tubes out, measure a.c. volts of the PT windings unloaded to see it's doing what it should all by itself. Then add a load, starting with just preamp tubes.

Bias voltage should be right before ever plugging in the output tubes. Current can be checked afterwards.

[eleventeen]

Off a bias tap, -91 volts is really "high". Understanding we are talking about negative volts. Load or no load on the main B+.


Is that 27K resistor at the bottom of the stack right, with good ground?


Maybe you're right about the tranny, it's just that trannies usually don't fail exhibiting low volts.

[p2pAmps]

I pulled the tubes and getting weird voltages on the rectifier.  Pin 4 200v and pin 6 145v

[shooter]

weird voltages on the rectifier

did you measure for AC also?, possibly only rectifying by 1/2

[sluckey]

I pulled the tubes and getting weird voltages on the rectifier.  Pin 4 200v and pin 6 145v

That ain't good. Suspect the PT. Do you get the same voltages on pins 4 and 6 with the recto tube pulled?

[p2pAmps]

I pulled the tubes and getting weird voltages on the rectifier.  Pin 4 200v and pin 6 145v

That ain't good. Suspect the PT. Do you get the same voltages on pins 4 and 6 with the recto tube pulled?

nope...   Pin 4 200v and pin 6 145v

pin 2 4.5vAC

[sluckey]

I pulled the tubes and getting weird voltages on the rectifier.  Pin 4 200v and pin 6 145v

That ain't good. Suspect the PT. Do you get the same voltages on pins 4 and 6 with the recto tube pulled?

nope...   Pin 4 200v and pin 6 145v

pin 2 4.5vAC

Huh? That's exactly the same.

[p2pAmps]

I pulled the tubes and getting weird voltages on the rectifier.  Pin 4 200v and pin 6 145v

That ain't good. Suspect the PT. Do you get the same voltages on pins 4 and 6 with the recto tube pulled?

nope...   Pin 4 200v and pin 6 145v

pin 2 4.5vAC

Huh? That's exactly the same.

No, when I pulled the tubes Steve and checked voltages thats what I got.  Tomorrow I'm going to tap in a new Classic Tone Tweed Bassman tranny I have for another amp and test it out.  I am wagering I have a bad PT but I've been wrong many times before...

[P Batty]

I've  seen more than one Pro Reverb with a replaced power transformer and they have often been converted to a SS rectifier. The PT might have been a little underrated?

[sluckey]

No, when I pulled the tubes Steve and checked voltages thats what I got.

Sorry, I was not listening to what you were saying. 

[p2pAmps]

No, when I pulled the tubes Steve and checked voltages thats what I got.

Sorry, I was not listening to what you were saying. 

LOL, no worries.  Yesterday was Big Green Egg BBQ day and...., well a few beers too :)

[Willabe]

Yesterday was Big Green Egg BBQ day and....,

Big Green Egg BBQ!!!!!!   

[p2pAmps]

Yesterday was Big Green Egg BBQ day and....,

Big Green Egg BBQ!!!!!!   

Oh Yeah baby

[Willabe]

Food looks great! 

Whats the little box in the bottom right hand corner? Temp read out from temp probes?

[sluckey]

That's a green egg stomp box. 

[Willabe]

That's a green egg stomp box. 

             

[eleventeen]

It's a phaser. Keeps all those kebabs in phase. See how they are all aligned? Mike omitted the teflon tubing over the spears this time, per customer request.

[Willabe]

, per customer request.

   

[p2pAmps]

We were right Sluckey, it was the PT.  I stuck a new Classic Tone I had in there and she straightened right up.  Good strong amp now.

[sluckey]

Alright. Now go show Timbo how to polish a chassis. 

[p2pAmps]

Alright. Now go show Timbo how to polish a chassis. 

Ha!  Timbo - DA sander and 80 grit...  Takes 5 minutes

[p2pAmps]

https://www.youtube.com/watch?v=LvYsYR_Kc7I

[mresistor]

That is very nice sounding Pro.  ;-)