Oliver Sound G-300 Amp Head.....

[Jack_Hester]

A couple years ago, I acquired an Oliver G-300 amp head.  The owner was a collector, and said that he had never seen one like it before, or since. 

It has a 6DR7 and a 6EU7 on a stand-alone Reverb board, both checked strong on my B&K 707.  The 6AN8A was bad, but I had the good fortune of having only one, in an old tube caddy.  It was very strong, so in it went.  There were two more 6EU7's, one very strong, and one weak on one triode.  Replaced it with a very good one. 

The two power tubes are 7027A's, both fairly new JJ's.  I had two Wurlitzer's that I could have used, but one of them was about 85%, and the other 100%.  Probably wouldn't have mattered, but I put the JJ's back in. 

The amp has two channels, two inputs each.  Channel 1, Input 1, is high gain, Input 2 being low.  Same for Channel 2.  Channel 1 has a Tone Stack with Bass, Treble, Mid-Range, and Ultra-High (as they are labeled).  Channel 2 has just Bass and Treble. 

Channel 1 has Reverb (with an Intensity pot), and Tremolo (with Speed and Intensity pots). 

Other than the Power Switch and the Anti-Hum Switch, that's it.  There are two connections on back, both 1/4" Phone Jacks.  One is hand-marked 'Speaker', and the other is unmarked.  But, after looking inside, I found it to be the jack for the footswitch.  I had a dual footswitch that I picked up from Doug, for such an occasion.  I pressed it into service here.

I warmed the amp up, and set all pots (but the two Volumes) to 12 o'clock.  With Channel 2 at minimum, I rolled Channel 1 Volume to full, no connections to the inputs.  Other than hiss above 3/4, a very quiet amp.  Only a very subtle hum, if you put your ear to the speaker.  By the way, I had an 8", 8 ohm speaker connected, though I have no idea what the OT impedance ratio is. 

With Channel 1 at minimum, I rolled Channel 2 to full, no connections.  Same as Channel 1, but even less hiss. 

Let me note here that the 9-pin tubes have no more than maybe 2" between them, and no shielding.  The power tubes might have 1", and the rectifier (forgot to mention that it is a 5AR4/GZ34) is about 1" from one power tube. 

Back to Channel 1, Input 1, small Supro guitar connected.  Volume on guitar at full, Tone to full Treble, amp volume at 12 o'clock.  Reverb on.  Reverb is really nice, from subtle to very full.  Reverb off, and Tremolo on.  Tremolo is likewise very nice, from a medium speed to very fast, and Intensity from almost inaudible to strong.  Footswitch works on both.  Thanks, Doug.

Next, Channel 1 at minimum and Channel 2 at 12 o'clock.  This is a Bass channel.  Not as bright as the other, but still good enough for guitar.  I played with the Tone controls on this one a bit more.  Good deep Bass to very good Treble.  Again, not as bright as Channel 1 can be made. 

This is one good sounding amp, even in an 8" speaker, though the speaker distorts badly if the volume is more than 12 o'clock.  Once I determine what the impedance ratio is, of the OT, I'll connect a proper 12" speaker. 

I have some socket adapters that have a test terminal at each pin.  I'll remove the tubes and connect a cable to the speaker jack.  Then, I'll connect my auto-transformer to pin 3 of each power tube and put a meter on the speaker cable.  I'll increase the voltage on the primary of the OT, until I get 1 volt on the secondary.  I'll run the numbers and calculate the impedance ratio.  The effective load resistance (RCA) from plate to plate is anywhere from 6000 to 6600 ohms, assuming that I have measured voltages in the 400 to 540 volt range, as per the sheet.  I'll put it all back and leave the socket adapters in place, to get some voltage readings with the 8 ohm speaker attached.  I also have a Bias test adapter, so that I can see the bias current with this speaker attached.  I can connect any combination of speakers to get the correct secondary impedance needed, once I've done all the above.

Anyway, all that being said, has anyone heard of this amp before?  With the busy construction of this amp, it would be very labor-intensive to draw a schematic.  I just don't have the time right now.  I've got literature (in PDF format) on several Oliver amp models.  But, none like this one.  I truly believe that I may have a one-of-a-kind prototype.  And, I only dug it out last night, to take to Church to show to my buddy Don, as we were having a pot-luck supper and games.  I didn't want to play games, and neither did he.  So, I brought show-and-tell.  The more we looked, the more we realized what I had.  Had some other chores to take care of this morning, but this has occupied my thinking, otherwise.  And, it sure has been a fun venture.  Hopefully, it's going back to Church next Sunday, with the proper speaker cab.

Please drop me a note, if you have, or find, any literature on the amp.  Thanks for the help.  I'll post anything that I come up with.  Have a good one. 

Jack

[Willabe]

That's a great 'show and tell' for pot luck church dinner!   

(I don't like playing games afterwards latter at dinners/party's either, although my wife does.)

Yes, please keep us posted if you find anything more on it Jack.

            Brad     

[Willabe]

Hey just did a search and found an ebay sale for a Jess Oliver Sound G-300, B-15 proto type amp, $859.


                 Brad         

[Jack_Hester]

Hey just did a search and found an ebay sale for a Jess Oliver Sound G-300, B-15 proto type amp, $859.


                 Brad       

For some reason I can't find it.  Can you post a link to the one you found?  Thanks.

Jack

[Willabe]

I can't get the link, but copy and paste this into your search box, should come up. (The sale ended in Dec. 2013.)   

JESS OLIVER SOUND AMPEG G-300 GUITAR/BASS AMP PROTOTYPE-B15


              Brad   

[Jack_Hester]

Well Brad, this is embarrassing.  I still couldn't find it on ebay.  But, I did the same search in my saved emails.  There it was.  My buddy Don and I were trying to figure out when I got it, and to no avail.  Way less than a couple years, but that's the way my memory goes these days.  I'll tell Don, and he'll get a good laugh. 

But.  I'm still alone.  Thanks for doing the search, but we're right back to me.  Keep up the good work.  The only schematic we find may be the one that I post someday.  Just remind me that I did it. 

Jack

[Bub]

http://www.tubebooks.org/file_downloads/oliver_amps.pdf
Hi Jack,
I don't know if this is the info you already have but there are some schematics
Bub 

[Jack_Hester]

http://www.tubebooks.org/file_downloads/oliver_amps.pdf
Hi Jack,
I don't know if this is the info you already have but there are some schematics
Bub

Thanks, Bub.  Yes, I have this, but I'm glad that you posted the link.  Never looked at it close, until now.  Mainly because I'm handling the amp for the first time.  I have a model PA-100X, that is in that file.  Need to handle that one next.  The 300 doesn't have a hum balance pot like the 100X, but the two channels have a lot in common with mine.  When I look through all of the schematics, I'm seeing a trend.  Same tubes, and mostly the same configuration.  Now that I think about it, ain't fired the 100X up, yet.  It's about 2/3's the size of the G-300.  Both are some kinda heavy. 

The G-150/200R (in the schematic) may have the Reverb/Tremolo configuration like in my 300.  Even the amp section may be the same, though the PS is solid state, unlike the G-300.  Looks like I might have a start on my schematic. 

Wish I knew when he built the 300, in relation to the others.  And, why?  I'm thinking that I can draw up a schematic from these.  Then, do a point-to-point trace of mine, to verify and correct.  No time soon, but you've got me headed in the right direction.  Thanks, again.

Jack

[Willabe]

Well Brad, this is embarrassing.  I still couldn't find it on ebay.  But, I did the same search in my saved emails.  There it was.

I didn't do a search in ebay, I searched in my home web page search box. I think that because the auction is over/closed you won't/can't find it searching in ebay for it?

Anyway glad you found it Jack.


              Brad      

[Willabe]

I just looked at the link and it shows a G-300 model that's for a vibraphone instrument. Page 12.

           
               Brad     

[PRR]

>> found an ebay sale for a Jess Oliver Sound ....
> I can't find it. 

It's on eBay *Belgium*.

Link

I can not get the images, either FireFox or Chrome. If I enable scripts, I get a notice about cookies (which I don't think I am blocking).

[Jack_Hester]

Ok.  Took some readings. 

I only took a plate voltage reading on one 7027A power tube.  They are aligned front to rear and the chassis is made on a slope to the rear.  The front tube with the socket adapter won't fit.  Hits the cab top.  So, I took a voltage reading on the rear tube at Pin 3, as well as a voltage reading on the rectifier tube at Pin 8.

7027A - Pin 3 - 520vDC
5AR4   - Pin 8 - 522vDC

Then, I installed my Bias test sockets and took readings on both.

Front 7027A - 47mA (read as 47mV, from the meter)
Rear  7027A - 43mA (read as 43mV)

The data sheet (RCA) states a max plate dissipation of 35 watts.  The calculated safe Bias value for 70% of this is 47.1mA, at 520vDC at the plates.

From this data, I see no need to do further testing on the OT.  I believe 8 ohms secondary impedance is correct. 

I now need to connect the amp to a really good speaker.  And at some point, I need to find or work up a good schematic to go with it. 

Jack

[Jack_Hester]

Well, crap!  I thought that as soon as I had a good speaker to connect it to, I could button things up, and prepare it for the trip to Durham.  Not so.  The output has dropped to almost no sound. 

All tubes are heating.  Checked the same voltages as before, and they are still there.  If I shut everything off in the room, and roll the volume up (either channel), I just can hear the guitar.  I checked all the tubes again, and still have strong readings.  Checked the impedance on the new speaker, from the end of the cord: 7.2 ohms DC.  Got the little 8" speaker that I originally used, and connected it to the amp.  Same.  So quiet that you can't hardly hear the guitar. 

I really didn't want to troubleshoot an amp this afternoon, but this one's got me stumped.  Only a minor difficulty to open it up.  Think I'll wait until the cool of the morning, tomorrow early.  At least, with the chassis flipped over, I can make a voltage chart of all the tubes.  I'll have to assign labels and comment as to the tube location. 

Crap. 

Jack

[Jack_Hester]

Removed the chassis from the cabinet, and reconnected the spring tank.  Connected an 8 ohm speaker to the output, but no connections to the input.  All controls at 12 o'clock, with the volumes set to max.  Here are some pictures of the amp:









I edited two of the pictures to put labels by each tube.

I powered up the amp and let it warm for about 10 minutes.  Then, I recorded voltages for each tube.  Note: The line voltage to the PT was 123vAC.  I have attached my voltage charge.  If you will look at the schematics in Reply #6 (Thanks, Bub), you can get an idea of what each tube is doing. 

Without doing a complete hand-trace of the circuit, here's how I think that the tube assignments are:

V8 - 5AR4 - Rectifier
V7 - 7027A - Power
V6 - 7027A - Power
V5 - 6EU7 - 1/2 Tremolo, 1/2 Reverb
V4 - 6DR7 - Reverb
V3 - 6AN8A - Gain (pentode), PI (triode)
V2 - 6EU7 - Channel 2 Pre-amp
V1 - 6EU7 - Channel 1 Pre-amp

From my voltage chart, and using the PA-100X as my reference schematic, I'm wondering why I have such a high voltage on V3-Pin 3 (cathode of the triode).  It matches Pin 2 (grid of the triode). 

Now, I'm going to dig through my tubes, and see if I can come up with another 6AN8 or 6AN8A.  Please look this information and drawings over, and comment please.  Thanks for any and all help.  I'll be back.  Have a good one. 

Jack

[sluckey]

I'm wondering why I have such a high voltage on V3-Pin 3 (cathode of the triode).  It matches Pin 2 (grid of the triode).

Because it's a split phase PI. The grid is likely directly coupled to the plate of the pentode since you have the same voltages. And the cathode will have a voltage that should be a little volts more than the grid.

[Jack_Hester]

Thanks, Steve.  Looking at the other schematics, the 6AN8A is used in the same configuration for each (that is, the ones that use this tube).  And, the grid of the triode is directly coupled to the plate of the pentode, in the other schematics.  Confirmed this on mine.

I'll rule that out as a possible problem.  Back to troubleshooting.

Jack

[sluckey]

Here's the schematic for my Sunn that uses a 6AN8 for the PI. Bet the two circuits are very similar. Ampeg used the 6AN8 in some of their stuff but they were more fond of the 7199, which is similar to the 6AN8.

http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

[Jack_Hester]

As I'm short on 6AN8's, I opened up my PA-100X and borrowed that one.  This is another amp that hasn't been powered up, so it's waiting it's turn. 

Let me back up a bit.  I made myself a 'listener' by using my powered headphones connected to a jack that I have leads and alligator clips attached.  I attached the barrel lead to the chassis, and clipped a small .022uf cap to the tip alligator.  I put my rubber glove on my testing hand and used the cap to trace the signal.  I used my little Supro guitar as a signal source.  I tested both pre-amps and traced a good strong signal all the way to Pin 7 on both V1 and V2 (both 6EU7's).  I then found a bit weaker signal on V3-Pin 8 (6AN8A).  I attribute the drop to the preceding tone stacks.  However, no signal was on Pins 1, 2, or 3 (triode) of this same tube.  This tube tested very strong (and still does). 

So, here's where I swapped it out with the borrowed tube, from the 100X.  Now, I have a signal at Pins 1, 2, and 3, of about the same strength (listening strength) as on Pin 8.  And, I'm now hearing a very weak guitar sound out of the speaker.  I would have thought that V3-Pin 2 should have a much stronger signal than Pin 8.  Not so, here.

One good lesson learned.  A tube tester ain't an analyzer.  Just because a tube will test good doesn't mean that it will work. 

I'm tired (brain, and otherwise).  I'll pick up on it again early in the morning.  Thoughts, please. 

Have a good one.

Jack

[Jack_Hester]

Decided to check the OT while I'm scratching my head, troubleshooting.

From the P-500 schematic, I confirmed the Secondary wire colors and the corresponding impedance. 

Black - Common
Yellow - 8 ohms
Green - 16 ohms

Though I didn't see it in the schematics, my Primary wire colors are:

Red - Center Tap
Blue - V6-Pin 3
Brown - V7-Pin 3

I have confirmed from V7-Pin 5 (7027A) to V3-Pin 1 (6AN8A), by way of a 1.5K resistor and a .1uf capacitor.  When I make my own schematic, I'll assign labels to all these components.  I have confirmed the same for V6-Pin 5 (7027A) to V3-Pin 3 (6AN8A), by way of a 1.5K resistor and a .1uf capacitor. 

Removed the power tubes.  Got out my USN Auto-transformer and connected it to the OT Primary leads (Brown and Blue).  Connected a meter to the same.  Connected another meter to the Black and Yellow Secondary leads.  Fired up the a-t and increased the voltage until I had approximately 1vAC on the Secondary.  Here are the results:

25.8vAC In -> 1.05vAC Out

When I run the numbers for a Secondary impedance of 8 ohms, I get:

Primary impedance - 4830 ohms
Turns ratio - 24.6
Impedance ratio - 603.8

Next I clipped the meter to the Black and Green Secondary leads and read the following:

25.8vAC In -> 1.48vAC Out 

Running the numbers for a Secondary impedance of 16 ohms, I get:

Primary impedance - 4862 ohms
Turns ratio - 17.4
Impedance ratio - 303.9

Again, from the 7027A tube spec sheet: Fixed Bias - 2 Tubes - Effective Plate Load Resistance (Plate to Plate) - at my plate voltage of ~510vDC, to get the ideal desired Primary impedance (approaching 6600 ohms) on my OT, I should have a Secondary impedance of 10.5 - 11 ohms. 

I'm not going to worry of the difference.  The main goal is to get it back to working.

Anyway, that's where I am.  Nothing fixed, as of yet.  But, I'm building my documentation for the amp. 

Jack

[Jack_Hester]

Well, time for a break.  But, I've managed to rough out a layout of the mainboard, though I've not confirmed the drawing with the actual board.  This is a view from the underside.  When I'm satisfied that I've got all the points in place, I'll invert it and draw in the components.  At this point, it's looking very much like the PA-100X mainboard. 

Anyway, more to come.

Jack

Note: 05Sep14 - See Reply 39 for complete drawing.

[Jack_Hester]

Here's the schematic for my Sunn that uses a 6AN8 for the PI. Bet the two circuits are very similar. Ampeg used the 6AN8 in some of their stuff but they were more fond of the 7199, which is similar to the 6AN8.

http://home.comcast.net/~seluckey/amps/sunn/sunn_sceptre_1971.pdf

Thanks for the comparison.  I believe that if I were building a modern version of the Oliver, I'd split the tubes out to an individual pentode and 1/2 of something in the 12A_7 family. 

So far, these non-standard tubes (by modern standards) are still fairly easy to come by at reasonable prices.

Jack

[Jack_Hester]

Well, I've been a bit busy, documenting this amp.  Far from finished, as I have considerable cleanup to do on the layout.  I've drawn the amp schematic, but not the Reverb/Tremolo portion. 

I used my PA-100X schematic to make the initial drawing.  Then, I made a drawing of the actual main board layout.  Both are on the attached drawing (huge size A, for high resolution). 

Note: My components are color-coded for my ease in reading.  Capacitors are Blue and Resistors are Red.  If components are unconfirmed between the board and the schematic, I color them to be black if printed on a white background.  Or white, if printed on a black background. 

Next, I will make a drawing of the Reverb/Tremolo board, and then a schematic of the same.  Then, like the main amp/board, I'll reconcile the two. 

You'll note on the schematic and board that there are unidentified components.  The caps, I couldn't read.  But before I finish, I'll try to get a reading with my cap tester. 

While doing all this, I've been thinking about things that make be dragging my signal down.  The first check that I came up with is to lift the two wires that tie the signal path from the main amp, to the Tremolo and Reverb.  I'll pull the tubes (V4 & V5) and see what happens.  If not change, I'll lift the wires and give another try.  If this is not the resolution, I'll leave them lifted until the problem is fixed.  Then, tie them back in. 

Another Note: The Tone Stacks  for both channels remind me of the way Ampeg built theirs, on a card mounted to the pots.  In this case, they are very long cards.  Channel 2 came out very easily, as it was on the bottom.  So, when the chassis is inverted, I removed the knobs and nuts, and slipped the whole card out.  Channel 1 is a bit more buried.  But, it can come out with some difficulty.  I did not confirm the components on it, as I did not remove it.  I confirmed several on the Channel 2.  Couldn't read the cap values on most.  I'll give them a check with the cap tester, as they are easy to get to, mounted.

Anyway, take a look and please comment, if you see something that needs changing. 

Jack

Note: 26Aug14 - Updated drawings to reflect the Tremolo tie points.  Thanks Sluckey.

Note: 05Sep14 - See Reply 39 for complete drawing.

[Jack_Hester]

Another though came to me this morning.  The NFB may be an issue.  It has what appears to be a 2.2K resistor (Red-Red-Red).  Other schematics call for a 22K (Red-Red-Orange).  My poor eyesight leaves doubt, but my meter shows 65R.  That may be because I'm measuring while still in the circuit.  It may be because of a defective capacitor that is paralleled with it.  Easy enough to lift that wire.  Probably the easiest thing to access on the board.  Everything is so compact. 

Anyway, I'll post those findings a little while later.  Have a good one.

Jack

[sluckey]

but my meter shows 65R

Your meter is correct. There is a 68Ω resistor connected between one side of that resistor and ground. There is a 1-2Ω OT secondary connected between the other side of that resistor and ground. IOW, the 68Ω resistor is essentially parallel to the NFB resistor, so your meter will see slightly less than 68Ω regardless of whether the NFB resistor is 2.2K, 22K or 22M.

[Jack_Hester]

Your meter is correct. There is a 68Ω resistor connected between one side of that resistor and ground. There is a 1-2Ω OT secondary connected between the other side of that resistor and ground. IOW, the 68Ω resistor is essentially parallel to the NFB resistor, so your meter will see slightly less than 68Ω regardless of whether the NFB resistor is 2.2K, 22K or 22M.

Thanks for the response.  My drawing(s) are approaching accurate.  I couldn't wait until later.  I lifted the NFB wire and measured R44 (I'm now starting to reference my drawing labels).  Meter shows 1.9K.  So, my eyes are seeing the color bands fairly well.  I am seeing Red-Red-Red.  As another quick check to verify, I took a peek at the same resistor on my 100X, as I have the back grill off (when I borrowed the 6AN8A).  It's a Red-Red-Red, as well. 

I connected the speaker, Reverb tank, footswitch, and power.  With guitar on either channel, a very low volume at full.  Tremolo still works, as well as Reverb.  I'll pull those tubes and give a try.  If no change, I'll lift those connections to the main board.  Shouldn't be more than a couple. 

I will go ahead and make a drawing of the Reverb/Tremolo board.  Then, I'll have something accurate to troubleshoot that circuit, if needed.  If someone will take a look at the G-150/200R drawing (posted by Bub in Reply #6), please explain how the Tremolo sends it's oscillation back to the signal path.  I'm missing something that may become clear, when I make my drawing.  Thanks.

More to come.

Jack

[sluckey]

The LFO causes the light source inside an optocoupler to blink at the trem speed. The blinking light causes the LDR side of the optocoupler to vary its resistance at the trem speed. See points labeled "T"? Now look up at channel 2 preamp and you'll see points "T" directly in the signal path, between a coupling cap and the tone stack. The LDR is in series with the signal. This operates differently than a shunt LDR circuit as found in Fender AB763 amps.

If the LDR fails, there will be a very high resistance between points "T" and this will cause the signal to be very weak. Put a short across points "T" and see if you have good volume.

[Jack_Hester]

The LFO causes the light source inside an optocoupler to blink at the trem speed. The blinking light causes the LDR side of the optocoupler to vary its resistance at the trem speed. See points labeled "T"? Now look up at channel 2 preamp and you'll see points "T" directly in the signal path, between a coupling cap and the tone stack. The LDR is in series with the signal. This operates differently than a shunt LDR circuit as found in Fender AB763 amps.

If the LDR fails, there will be a very high resistance between points "T" and this will cause the signal to be very weak. Put a short across points "T" and see if you have good volume.

Thank you ever so much.  I completely overlooked the 'T' tie points in that channel.  Now it makes sense.  By the way.  My channels are the opposite from the other schematics.  My Channel 1 is actually Channel 2 in each of Oliver's drawings. 

At the moment, I have the chassis up on edge, to draw the Reverb/Tremolo board.  In a minute, I'll flip to turn the underside up and locate those points.  I'll be back. 

Jack

[Jack_Hester]

Steve -

Found the 'T' tie point in my Channel 1.  It is on the Tremolo Intensity pot.  C1 is connected, by means of a shielded wire, to terminal 3 of this pot.  Terminal 3 is jumpered to terminal 2.  The front end of the Tone Stack is connected to terminal 1 of this pot.  The LDR is tied across terminals 2 and 1. 

I fired the amp up and tried it as it was before.  Then, I jumpered terminals 2 and 3.  The oscillation in the speaker stopped, but no increase in volume.  Even though this does not affect my Channel 2, I gave it a try again.  Volume still at the same level as Channel 1, both tested at full volume. 

I'll go back and amend my drawings to reflect the tie points.  Thanks for looking and I'll post more as I complete the next drawing.  Have a good one.

Jack

[Jack_Hester]

Note: 05Sep14 - See Reply 39 for complete drawing.
Jack

[Jack_Hester]

Ok.  First draft of the Reverb/Tremolo board.  Nowhere near as busy as the Mainboard. 

Next, I'm drawing the schematic for this board, and then populating the board drawing and correcting/confirming the schematic, accordingly. 

Again, this is plotted to an A0 sheet size, for resolution.  The drawing is to scale and is only 4-7/8" x 3-7/16".  I'll be back.

Jack

Note: 05Sep14 - See Reply 39 for complete drawing.

[Jack_Hester]

Well, the Reverb / Tremolo board is drawn, and the schematic updated and reconciled with the boards.  There are some components in this amp that are not on the other Oliver Amp schematics.  On my drawings, the component values that I could not identify, I indicated with double question marks ??.  Otherwise, everything has a label, boards and schematics.  If you see any discrepancies, please note and respond.  I'll make those corrections and re-post.  I'm sure there are some that haven't jumped out at me.  But frankly, I'm tired of drawing.  It had to be done, so that I can now troubleshoot this fine old amp. 

I will remove the other drawings, as they are incomplete, or have errors.  This one is way more accurate, and I will update it as I troubleshoot and see errors. 

I'll take a break for awhile, but will begin the process soon.  Please comment.  Have a good one.

Jack

Note: 05Sep14 - See Reply 39 for complete drawing.

[sluckey]

Drawing a schematic and pcb layout AND reconciling any differences between them can be tedious. But, I've always believed that the intimate knowledge you gain about the amp during that process is more valuable than the time and energy it took to do so. I wish more people could realize that.

[Jack_Hester]

Drawing a schematic and pcb layout AND reconciling any differences between them can be tedious. But, I've always believed that the intimate knowledge you gain about the amp during that process is more valuable than the time and energy it took to do so. I wish more people could realize that.

That is a true statement.  The other Oliver schematics were a valuable help in building mine.  But, they still left a lot to the imagination.  I have little doubt as to what goes where, now.  And, I can almost trace from memory, though that won't last long.  Good to have it on paper (electronic, that is).

Jack

[Jack_Hester]

Working late for the next week or so.  But, I did manage to check some of my B+ voltages, just to be sure that everything is stable.  My B+4 was 436vDC, where it connects to R32 on V3 (6AN8A).  However, looking back at my voltage chart, and taking another reading at Pin-7 (meter still on the high scale), I'm reading zero.  Shouldn't I be seeing something more than this?   Tonight, I'll pull that tube and take another reading. 
 
Also, I was able to verify the value of C14 (same tube) to be .1uf.  And, checked again to see where the leads land.  My drawing still looks correct, but I'll see if I can lift the board a bit (many external wires attached) and make a better verification.  The other Oliver schematics show this cap going to signal ground.  I can't see this as being so, but I'm having doubts.  The crowded components and not easy access to both sides at once, makes this difficult.  But, I'm persistant. 
 
Anyway, I'll stay at it, a bit at a time.  Thanks for any and all suggestions.  Have a good one. 
 
Jack

[sluckey]

My B+4 was 436vDC, where it connects to R32 on V3 (6AN8A).  However, looking back at my voltage chart, and taking another reading at Pin-7 (meter still on the high scale), I'm reading zero.  Shouldn't I be seeing something more than this?

Pin 7 is the screen and should be about the same voltage as you see on pin 6 (plate). C14 shorted or R32 open are the most likely suspects. This is very likely the cause of your very low volume.

Your 6AN8 circuit is almost identical to the Sunn circuit I linked to above. Your C14 is connected exactly the same as the Sunn circuit. And your C13 is a 12pF (that's right, 12pF) in the Sunn circuit. Sunn did a direct copy of the Dynaco Mark 3 power amp. I wonder if Oliver borrowed from the Mark 3 also.

[Jack_Hester]

Thanks, Steve.   

I was hoping to do a bit more troubleshooting tonight (Sat), but I'm whipped.  Long day.  But.  This will be my first point to check.  I'll lift C14 on the downstream side.  I'll take a direct reading on R32, and verify it's value.  If it's good, I'll power it up and try that out, with C14 out of the circuit.  I've got a replacement, if it is the culprit.  For that matter, I've got them both. 

I'll post my findings.  Thanks again, for the fine help.  Have a good one.

Jack

[Jack_Hester]

Steve -
 
I looked at your '71 Sceptre schematic some this morning.  I have to work this holiday weekend, so I have time before the day starts. 
 
Many, many similarities do my amp, and makes me feel confident about my drawing accuracy (component placement).  Also, it makes me wonder who modeled theirs by the other.  Not knowing the history of Sunn, and learning more about Oliver, either one could be the original.  I wouldn't put it past Oliver to model his after the Sunn, as he couldn't manufacture his own design.  Even though he owned the patent.  Ampeg had the manufacturing rights.  His Oliver Sound designs are a departure from the Portaflex, though still very similar.  Thanks a bunch for that lead.  I have your drawing saved to my Sunn library. 
 
Jack

[Jack_Hester]

Pin 7 is the screen and should be about the same voltage as you see on pin 6 (plate). C14 shorted or R32 open are the most likely suspects. This is very likely the cause of your very low volume.

Your 6AN8 circuit is almost identical to the Sunn circuit I linked to above. Your C14 is connected exactly the same as the Sunn circuit. And your C13 is a 12pF (that's right, 12pF) in the Sunn circuit. Sunn did a direct copy of the Dynaco Mark 3 power amp. I wonder if Oliver borrowed from the Mark 3 also.

Lifted one leg of C14 and took a resistance reading of R32.  Looks like R32 is open, though no signs of burn or any other discrepancy.  Zero ohms on any scale.  Being as I have C14 halfway out, I think I'll replace it with a new one.  This one is rated 400vDC, and the replacement that I have is not only a bit smaller, but rated for 600vDC.  So tomorrow, if time permits, I'll pull C14 and R32, replacing both.  My 1M resistor is a metal film. 

Hopefully, I'll have good news after that.  More to come.  Have a good one. 

Jack

[Jack_Hester]

Took a bit of time after I got home this evening, to remove and replace both C14 and R32.  Connected all cables and warmed the amp.  I suppose that I should have taken voltage readings, at that point.  But, the speaker came alive for the first time, in a while.  Mainly the noise was from the open chassis sitting under the bench fluorescent lamp.  Very loud hum, as the volume pots were at full.  Switched the lamp off and the amp became remarkably quiet.  And, no shields on any tubes.  I will try to remember to snap an overhead picture with a 6" ruler next to the tubes.  They are close. 

And, the 1M resistor still reads open.  I twisted the leads a bit, but no change.  And, it has no outward appearance of being defective.

So, I turned the volumes down, put all other pots at 12 o'clock, and retrieved my little Supro guitar.  Tried each channel at half volume, only strumming chords, and my wife tells me that I've got to take that out of the house.  I now have my amp back.  And, better.  I don't think that it was this loud before.  Tremolo has good range and sounds good.  The Reverb sounds really good, too.  I will wait until I have it all back together, before playing with the Tone controls.  It goes to Church for sure, this Sunday.  The speaker that I'm using is a 12" Eminence Wizard, 8 ohms impedance, that I put in one of my really nice Bell & Howell speaker cabinets.  This cabinet will go with the amp when I carry it on Sunday.  I will keep it fairly active through Saturday, just to be sure no other issues arise.

Tomorrow, I will take complete voltage readings and save them to the voltage chart on my drawing(s).  I'll also take another set of bias readings.  I'll post the updated drawings, by the weekend.

Thanks ever so much, Steve, for your troubleshooting input.  Cut a lot of time off of my having to do so, with limited time to devote to it.  Big payoff. 

Thanks to everyone else for your comments, links, and for following this thread, ready to give your input as well.   I'll have a little more to post, but hopefully, this one is on it's way to being done. 

Jack

[Jack_Hester]

Well, finished up the amp this morning, by taking a complete set of voltage readings, and reinstalling the chassis into the cabinet:







And, the original picture of the front, before repairs:



Channel 1 does not have the gain that Channel 2 does.  I'm thinking that it's because the signal passes through the LDR, before heading for the next stage.  I did not test either above half volume, this morning, and they are both plenty loud.  Wish that I had thought to take a picture of the LDR/Lamp holder.  Appears to have been made from a pistol cartridge case.  The LDR is glued into a drilled hole in the case bottom.  The lamp is fitted into a hole, drilled across the case, and very close to the LDR.  The end is crimped flat and maybe soldered.  Some kinda neat setup. 

I've attached the latest drawing with voltages.  Revision is the same as I haven't had to change anything since the last. 

One thing I did do is add a thermistor to the primary of the PT, a bit along the Ampeg influence.  My secondary voltages dropped a bit, but I'm well pleased with the overall sound of the amp. 

I've got a little work to do to the layout drawings, mainly to align the Tremolo/Reverb board with the Mainboard, as it is in the amp.  I'll post that, when it's complete.  Other than that, I think I'll start enjoying it. 

Thanks again, for the help.  Saved me considerable time troubleshooting.  Have a good one.

Jack

[Jack_Hester]

I'm reviving this topic just to add a bit of an update.  I carried this amp to a friend at Church, Don, who is our music leader.  He and I had both pretty much forgotten about this amp.  Nasty weather gave him an opportunity to dig it out and give it a good shake down.  Here's some of his comments: 

The test results are in, and it sounds great.  Extremely versatile EQ.  I don't think the ultra high did anything because it's above the frequency range of the speaker.  Doesn't matter.  The reverb is very deep.  The tremolo works great, but I wouldn't mind if it would go a little slower.  I'm guessing some combination of resistors and capacitors determines that.  That's a very minor quibble.  Over all, it's a real winner.  I'll bring it back Sunday.

My first trial when I have it back home is to connect it to my Oliver Orbital Sound Projector and an old speaker cab that has a new Fane speaker in it.  I believe this speaker has a fairly wide frequency range.  I'll see how it sounds with that. 

Anyway, I report the results.  Have a good one. 

Jack