Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jack_Hester on October 24, 2011, 08:15:46 pm
-
Awhile back, I bought an old unidentified PA amp and speakers. Just something to study. I emailed the seller to remove the tubes and package separately. And, to fill the inside of the cab with some heavy packing. Well, they packaged the cab securely in a box, but did not do as I asked. The PT ripped itself loose from the chassis and proceeded to destroy the tubes and rip some of the guts out. And, tear the chassis loose from the cab and break one of the cover latches. I took these pictures to send to the seller. Now that it was no longer usable, I negotiated a settlement where I could keep it all for the cab, chassis, and speakers. We settled, and this is what I have:
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/Brokenamp02.jpg)
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/Brokenamp03.jpg)
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/Brokenamp04.jpg)
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/Brokenamp05.jpg)
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/Brokenamp06.jpg)
I stripped all usable components and boxed them all up. I straightened some minor bends in the chassis, and repaired the cover latch to it's former working order. So, what I have now is an empty chassis with 7 octal sockets, and a fine old cab. There's still a bit more repair to do on the chassis mounting holes, but that's easy work. I'll try to snap some pictures of the cleaned up chassis/cab, tomorrow afternoon. It looks way lots better than it did. And very usable. More to come.
Jack
-
Its funny (well not at all really :BangHead: ) but you have to wonder sometimes if it wouldn't be better if they didn't post 'fragile' on anything. It almost makes them want to try that much harder to see if it really is. Sorry to see the carnage, after being in shipping and receiving for most of my career its not uncommon (matter of fact, its super common unfortunately). For whatever reason, pride of work doesn't apparently mean anything to people (well in this case, we're talking a seller, but even the guys who do it for a living don't seem to care, let the insurance sort it out is the mentality). I've packed things where I was sure they couldn't damage them. Crated, cross screwed wood supports bolted to pallets, you name it. Doesn't seem to matter, make it harder for a average gorilla to destroy and they'll just find bigger gorilla's! :icon_biggrin: I wish you luck with the remnants, it looks like you'll have something to work with at least. I hope your deal worked out to something worthwhile.
Regards,
Don
-
I apologize for not not posting the latest pictures. I'll upload them to photobucket this evening and link them. The cab and chassis actually looks very good, now that the broken components (and all the rest) are removed, and the cover latch repaired. The two microphone channels use a single triode tube, each, that are under the hood. The record channel has it's own, outside. I'll probably use dual triodes for each, just for the extra gain. The rectifier tube was some variation of a 4 pin tube. I have an 80 stuck in the socket, for now. I may change the socket for an 8 pin.
I crated a restored Flathead 45 engine for shipping from the East coast to the West. 2x4's, screws, gorilla glue, heavy plywood box, through-bolts to hold the engine mount to an equally heavy duty pallet. The carrier managed to destroy the crate. The customer filed for the $3K insurance and collected. Picture looked really bad. It took a serious tumble. I'm not doing those kind of builds anymore (only because I'm not doing restorations like I did), so I'm not faced with that problem anymore.
Jack
-
I straightened some minor bends in the chassis
was UPS the carrier?
-
Yes.
-
Yes.
figures... they are brutal. :sad:
they dinged an amp shipped to me form <300miles away... it was wrapped in bubble wrap and packed in peanuts - they still managed to get a corner.
-
Ok. I managed to snap some low-res pictures. I have a camera that is way too high-tech for me, and had to reset it to default settings. I forgot to go back and reset the images to high-res. Anyway, here is the chassis/cabinet, ready for planning a build:
Cabinet with cover intact:
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/CabinetFront05.jpg)
Repaired latch:
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/CabinetSide06.jpg)
Chassis front with tubes inserted in the sockets:
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/ChassisFront02.jpg)
Chassis top (note the single triodes in the enclosure):
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/ChassisTop03.jpg)
And, chassis bottom, with all components removed:
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/ChassisBottom04.jpg)
I haven't given much thought as to what I would like to build, but something like an all-octal Bassman is one possibility. Maybe I'll have some time to ponder it next week. Back on night shift again, for 7 nights. Acid cleaning boiler tubes on our Unit 1. Hopefully, I can sketch some form of schematic and layout for the chassis. Nights are killer.
Anyway, more to come.
Jack
-
> chassis bottom, with all components removed:
:sad2: :sad2: :sad2:
What was the original tube line-up?
Where did the "transformer" under the chassis connect? To power-tube grids? Do its windings still have any ohms?
-
love the D getter GE 6L6GCs...
try to duplicate the original cktry... PRR may stop crying. :sad2:
:wink:
-
> chassis bottom, with all components removed:
:sad2: :sad2: :sad2:
What was the original tube line-up?
Where did the "transformer" under the chassis connect? To power-tube grids? Do its windings still have any ohms?
The 2 single triodes 'under the hood', one metal case tube in front (I forget the number), and the 2 6L6's are the only original tubes. The 6L6's actually came out of their sockets and remained intact. I put 2 tubes in the middle back from some of mine, for appearances and possible use. The PT crushed the 4-pin rectifier and one glass tube beyond recognition. The rectifier that I put in is an 80. The only terminal strip in the amp was under the hood with the triodes. All else was soldered either to tube sockets or lead-to-lead. There is a footprint of a very large PT, that was there originally. The one that broke loose had only 2 screws (sheet metal screws) holding it, both of which pulled through. When the PT came loose, it pulled a large number of components with it, mostly separating the leads from solder joints. I save all, unsoldering rather than clipping. I am not experienced enough to reconstruct such an amp, with components swinging. So, I opted to build something, leaving the appearances intact, as much as I could.
Directly under the PT, and out of sight, was the OT. A choke was also mounted under the chassis on the back. The OT leads are intact, as well as the choke. The PT leads are pulled through the paper wrap and bare wiring (shellac stripped) showing. The only reason some of the leads are still on is that the PT couldn't come out of the box, and the components below broke loose and were stopped by the chassis hole being too small to pass them. I saved all of these in a separate box from the components.
love the D getter GE 6L6GCs...
try to duplicate the original cktry... PRR may stop crying. :sad2:
:wink:
I don't know if they are still good. I haven't tested them. I plan to use them or replacements in the build for this project.
Not sure that I could have duplicated, with the damage underneath.
Jack
-
I managed to put something of a schematic together, from sketches that I made. I've never designed an amp before, so my first schematic is without component values. I'm trying to use as many of the existing components as possible. I believe the OT and the choke are intact and usable. The PT has wiring and winding insulation damage, and I don't want to risk it. I don't know if the 6L6's are good, but I have some that are, if these don't test well. For now, I'm planning to replace the rectifier socket with an octal. The original volume pots are 500K, as is the tone pot. Mostly, I see 1M pots in Fender amps, for volume. My GA-40 has 500K's for volume and a 2M for the tone.
Anyway, I've attached a .pdf of my first draft. Please critique and make corrections. Thanks for any and all help.
Jack
-
.
-
Thanks, PRR for looking at my schematic. Added a cap between the cathode follower and the tone stack. I'll go back and update my component tags tomorrow.
Jack
20Dec11 - Updated component count, making new capacitor C10, incrementing the remaining capacitor count by 1.
22Dec11 - Appended first draft of BOM to drawing.
-
setchell carlson - can't make out the model #. one on fleabay i bid on but lost. cool looking amp. should make a nice tote-a-long amp...
http://www.ebay.com/itm/270848053865?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649 (http://www.ebay.com/itm/270848053865?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649)
--DL
-
Updated schematic to reflect new component count. As soon as I can work up a tentative BOM, I'll post that.
Jack
-
Updated the schematic, above, to append a first draft of BOM. The existing 4 pots on the chassis (3 volumes, 1 tone) are all 500K. My BOM component values are taken from the various schematics that inspired this build, and only 1 volume pot is a 500K. The other 2 volume pots are 1M and the tone is 2M. I plan to replace all with modern pots of the values listed. If I left the second 2 volume pots as 500K, the effect that I see right off hand is that the volume will be very fast from minimum to maximum, like a very coarse adjustment. However, I don't know what effect it would have on the tone stack, to use the 500K. Can someone take a quick look at these?
Also, I have a 650vAC-CT Hammond transformer that I believe is very close to the original PT footprint. The one that tore itself loose in shipping had only 2 mounts. The original footprint has 4. My PT is made to mount into a cutout on the chassis, but I plan to set in on 4 steel 1" standoffs and trim out the original hole to line up with the wiring. The original had a square cutout to allow the replacement wires to feed through. I will clean that up with a knockout punch. Once I get the chassis cleaned up and transformers fitted, I will need to find some form of wrinkle black to repaint it. Is there a standard chassis paint like this? Thanks for the help.
Jack
-
Well, the more I studied the original iron, the less I liked them. I wish that I knew the history of this amp, as the OT was not original to the amp. There was another footprint under it that was larger. Even the choke is questionable, as it too is smaller than the original mounting holes. So, I made the decision to use new iron. I mentioned that I have a 650v PT. Digging around, I came up with a 4H Choke that will work right where the last one mounted. The 4200Ω impedance OT that I have is too tall to mount under the chassis, so I decided to mount it just behind the input box on top. Here are pics of them mounted:
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/ChassisFront07.jpg)
(http://i132.photobucket.com/albums/q32/Jack_Hester/Guitar%20Amplifiers/ChassisBack08.jpg)
The choke is hidden under the chassis, mounted to the back panel. That's as far as I got today. I think that the chassis is ready to strip and clean up the dents and dings. Then, wrinkle paint. Still looking for the proper paint for this. Think I'll browse some of the old radio forums for such. I'll post what I find. Also, I'll go ahead and pick up the power supply components and some solder terminal strips.
Jack
-
Rustoleum wrinkle spray paint should be available at Lowes.
-
It seems that the same paint that I have on hand for motorcycle cylinders is some of the same used by the Old Radio crowd. They say the best results is without primer (I don't use it either, on cylinders) and baked at 325°F for 10 to 15 minutes.
Rustoleum wrinkle spray paint should be available at Lowes.
I'll drop by there on Friday and take a look at their wrinkle paints. Mine may not be any good, as I've had it for a couple of years. Thanks.
Jack
-
They also have a textured finish (forgot what it's called) that looks similar to a powder coat finish. I used some to touch up patio chairs. Then I tried it on an old chassis similar to yours. Looks decent and hides blemishes well. Here's the amp I painted. Did the transformers too. Just wire brushed, sanded, and sprayed a couple coats right over the rust. :icon_biggrin:
http://home.comcast.net/~seluckey/amps/rocky/rocky.htm (http://home.comcast.net/~seluckey/amps/rocky/rocky.htm)
Just an idea... You can get some different style bell ends for that transformer from Weber. Probably fit and will allow you to stand the tranny up like the one on the right in the second pic.
-
Thank you for reminding me once again why I stopped buying equipment of this type (or anything breakable) from Average Joe sellers online. I have received a few things that arrived at my doorstep intact only through the grace of God and not because they were packed properly. The average person is pretty clueless about packing.
Regarding shippers, I follow these rules. If it is imperative that the item arrive in good condition but it does not matter to what address it is delivered then use FedEx. If it is imperative that the item be delivered to the correct addressee but it does not matter what condition the item arrives in then use UPS. I have better luck with the much maligned USPS.
-
I was much disappointed, hearing broken glass when handed the amp package. If not for hearing it, I may have left the packages intact for some weeks, as work was rather busy at the time.
All was not lost. We settled easily enough. And, I was left with some cool speakers and a neat cab and chassis to build on. I'll post pictures of the speaker cabs.
Jack
-
Work these days only affords me brief spells in the shop. So, I decided to start fitting components, as I'm creating the layout as I go. The PT is wired in to two terminal strips. One of them is close enough to the rectifier tube socket that I was able attach the two diodes directly between the terminal strip (PT out) and the socket. Same for the 5vAC filament. Even though it is only 1-1/2" between the socket the terminal, I used some twisted pair, shielded cable to connect the filament supply. So, rectifier wiring is complete.
Components arrived for the rest of the power supply and bias. My friend Don gave me a locking pot (10KΩ) that I mounted near V3 and V4, to be used for bias. I have solder terminal strips mounted for the PS, that will put it under the choke (mounted to the back panel on the underside). No wiring yet, as I spend a lot of time standing and looking, trying to picture the best place to put things. Takes up a lot of my build time, but I guess that's part of the building, too. I've got Saturday off, so I'm planning early morning (way before daylight) amp wiring, and hopefully I'll have power supply and bias finished by lunch. Then, other shop work comes into play for the afternoon.
The camera is on the blink, again, so no pictures of the rough build/fitting. If I can't figure it out this evening or over the weekend, I get the wife to run it by the camera shop in Durham, next week. Just as well. Decided not to prep the chassis, until I was through scratching, marking, punching, and drilling. My automatic punch don't, so I've got a new one on the way. Punching with a manual punch and hammer makes small dents in the chassis. A bit of body work is in order, to do some smoothing, before stripping and painting.
I don't have an original GZ34, but I do have a JJ GZ34. Can I expect any difference in output of this tube, as compared to the other? Also, if the original 6L6's that came with the amp are damaged (haven't tested them yet), what kind of difference will there be in the JJ 5881's and OEM 6L6's. I know the originals are similar in specs. Got a couple of the these 5881's on hand. If they are close to original specs, that's money saved. I have several OEM of the remaining tubes needed for the build.
Thanks for any info and help.
Jack
-
Same for the 5vAC filament. Even though it is only 1-1/2" between the socket the terminal, I used some twisted pair, shielded cable to connect the filament supply. So, rectifier wiring is complete.
I would be concerned about using STP for rectifier tube filament winding. And I definitely would not connect the shield to ground. STP is 'usually' made for small signal voltages and may not be suitable for the full B+ voltage that will be on the filament wires. I have several different kinds of good quality Belden STP and it all has soft plastic insulation that I would not trust for high voltage.
Not all STP is made the same though. I've seen STP with teflon insulated conductors. Your STP may very well be up to the task. Anyhoo, give it a thought. Especially about not connecting the shield to ground. There's no benefit to shielding those wires and having a ground conductor in the same jacket with those B+ wires may invite the gremlins.
-
I would be concerned about using STP for rectifier tube filament winding. And I definitely would not connect the shield to ground. STP is 'usually' made for small signal voltages and may not be suitable for the full B+ voltage that will be on the filament wires. I have several different kinds of good quality Belden STP and it all has soft plastic insulation that I would not trust for high voltage.
Not all STP is made the same though. I've seen STP with teflon insulated conductors. Your STP may very well be up to the task. Anyhoo, give it a thought. Especially about not connecting the shield to ground. There's no benefit to shielding those wires and having a ground conductor in the same jacket with those B+ wires may invite the gremlins.
Good point about the STP. This stuff that I'm using is some high dollar scraps that I scarfed up back sometime around February. Short stuff, and a few long pieces, all thrown in the scrap pile, from a project. This stuff was made for a harsh environment. They spared no expense when they bought for the project that this cable was used on. All the guys in the shop know to give me a heads-up on anything like this, that may end with good scraps like this.
I'll lift the shield and tape it back. Here are the specs on it:
http://www.belden.com/techdatas/metric/88760.pdf (http://www.belden.com/techdatas/metric/88760.pdf)
I tested a piece at work, back in the Spring, using a meggar set at 1000 volts, even though the specs say 300v. The results were >2000MΩ, so I will keep a close eye on it. Because I used a Euro-style terminal strip for my PT leads, I can slip an ammeter in series and monitor the filament current. With the exception of the B+, I'm well below specs for the filaments.
I also plan to use the same for my 6.3v filament supply to the other tubes. The output tubes are each about the same short distance from the terminals where I have the PT leads connected, for this. The other tubes will be several inches away.
And, I will use it for any long runs of signal wire/pots that only require 2 conductors or less. A bit stiff, but free.
I'll post my progress after closing up shop Saturday. Thanks for the comments and suggestions. Have a good one.
Jack
-
It's been a while since I've posted any progress. I've been nibbling at it, as time permits, and got the amp up and running. I am pleased that it works so well, as it was a difficult build for me. Point to point, that is. Not the neatest build.
Anyway, I took it to my friend Don's, and let him keep it for the past week or so. His comments so far are that it's 'timid' for what he expects from 6L6's. With my GA-40 nearby, there's no comparison. "It's not loud enough at full volume to leave the room", as he put it. I changed the PI from my first layout, just to simplify the build. It has a very low hum, with all volumes turned down, but I believe that it may be the filament wiring that I have jumpered from V5 to V4. I'll not worry about the hum, until I have resolved the signal strength issue. I have included the latest drawing and BOM. The drawing includes a voltage chart. I forgot to record the Bias Voltage on my drawing, but managed to record the Bias Current of 40mA. Don made a comment, today at Church, to the fact that the bias may need some tweaking. I don't remember his reason for the comment, but will quiz him by email.
I would appreciate any comments and direction, if anyone can point out errors that stand out. Or, any improvements. Thanks for all help.
-
> I changed the PI from my first layout, just to simplify the build.
At a belated glance, that V4 cathodyne is WAY wrong.
You expect plate resistor to equal total cathode resistor(s).
You expect cathode near 1/4, plate near 3/4, of supply voltage. Assuming 360V, K=60-120V, P=240-300V.
18V at cathode is suspicious.
Maximum output level from this point must be less than 18V. But though you neglected to get 6L6 G1 voltages (but got 3361.9V on a 5T4?? Point-Nine?), it must be 40V-50V bias and surely over 35V needed to drive it to full output.
> not loud enough ...to leave the room
If you need >35V drive, and give it <18V, that's half voltage which is QUARTER power. Ten-watt?
Where to start? There's many ways to skin cats, and several ways to put this cat's skin on so it fits better. Without looking at the broader picture, you could do this to get the cathodyne less lame:
-
Thanks, PRR, for looking the schematic over. Mark (don't remember his username) over on Music Electronics gave me a similar fix, so I put 100K's in place of the ones that you have as 47K's. And, replaced R30 with a 1.5K. I also changed my NFB to a 33K, as I had one way too low in value. I assigned a tagname to the extra resistor, but don't remember what I named it. Really woke up the amp. Not as 'timid' as before. And, being as I had the unused triode in V4, I let the tone stack signal go to it, and let it feed the PI. What a difference.
Channel 1 has way too much gain. Channel 2 and 3 are just right. It is over at Don's for a shakedown, but when it comes home, I plan to disconnect the Channel 1 and carry it right to the tone stack input, where C10 (from the CF) connects. I'm hoping this will be enough to tame it down. It is way too loud above about 3 or 4 on the dial. Hoping to get it down to the same level as the other two channels. My friend Don had this to say about Channel 3, the 'bright' channel:
"Just the right amount of gain, and tone as sweet as honey."
Thanks for giving it a look, and please comment as to the changes I made, as well as your thoughts on my moving the Channel 1 signal. When I'm finished making changes, and am satisfied with the end result, I'll post the schematic and BOM. As usual, your comments are much appreciated. Have a good one.
Jack
P.S. -
I missed your .01uf for C10. I need to go back to my as-built BOM and see what I used.
-
> your .01uf for C10.
That's a NEW part not shown in your PDF.
You have tone-driver plate at some DC voltage. You have tone control which does not block DC but leaks DC to ground. You have cathodyne grid which needs to be up at some DC voltage.
You need blocking both before AND after this tone control.
There's other ways to do it. Might save one part, but won't be cheaper.
> 100K's in place of the ones that you have as 47K's. And, replaced R30 with a 1.5K
Those are great values for 12AX7.
You have 6SN7. Lower Mu. Needs more bias voltage. Cathode bias resistor should somewhat less than 1/Mu of the other resistors. 12AX7 with 100K+100K/100 is 2K or 1.5K. 6SN7 with 100K+100K/20 is 10K or 7K.
Voltages are clues.
I think you could try 5K-7K bias here. The difference will only be when LOUD. Might clean-up the 40W-50W zone, and get full use of the 6L6es and high supply voltage.
100K top and bottom is great drive for the large >300K grid-leaks we use on self-bias outputs. Fixed-bias is supposed to use smaller gridleaks, technically 100K (there's reasons we can cheat-up to 220K). As the gridleaks get smaller and the deep-bias amp needs more drive, we may have to reduce the driver's load resistors to get sufficient slam.
At 220K gridleak and good B+ to cathodyne, two 100K is fine unless you play trash/thrash.
-
Thanks ever so much for the explanation for the values. Still much to learn, and a rusty old head to put it in. I will make the changes, correct my drawing to reflect, and post all when it is done. The build was not one of economics, so I'd rather add components to improve the amp.
I've got two hum issues to resolve, knowing that they would be possible causes for such, when I built. One is a short run of twisted pair, from the filament of V5 (one of the 6L6's) to V4 (the PI and now additional gain stage). It is about 2" long, and is run direct, and not close to the chassis. Other leads are all over it, so I knew that there may be some hum. Turns out, it was a very low, almost background hum. I left it alone, until other amp issues were resolved. All but one other filament feeds are shielded, twisted pair. The other is way away from all else, between V2 and V3. About an inch.
The other hum came when I added the last gain stage, the unused half of V4. That tube is about 2 inches away from the PT, and no shield between them. I have a small piece of thin walled pipe that I plan to add, to shield this tube. Figured that I could use a canned capacitor chassis mount to hold it. Don't know if this is practical. But, it's what came to mind. I even thought to build a metal box over the PT, to see how that worked out. The hum is not too bad, but I plan to make it go away. Just to clean up the amp.
Anyway, your component values/additions are noted, and will be implemented right away, when I am once again in possession of the amp. As I have to work through the weekend, it will be sometime next week, before it will be back home. Again, your help is much appreciated, and will update with new schematic and voltages, when all is in place. Have a good one.
Jack