Trying to Match a Hammond PR-40 PT to a Twin Project;>/(Hot Shot 48 Plexi)

[Platefire]

sluckey, you must be having a senior moment----the 2204 and 1987 model Marshalls both have 5K Presence pots. Even the original Bassman that they were modded from has a 5 K Presence??!! Platefire

[sluckey]

There are more than one 1987 and 2204 amps, JMP series and JCM-800 series. Your presence circuit matches the JMP 2204, but not the others, nor does it match the 5F6A Bassman circuit. Sorry for my confusion.

[Platefire]

Thanks for the nice schematic comparison! What would make the 22K the one? Was it determined on later model's that 22K worked better or what? Platefire

[PRR]

> What would make the 22K the one?

The pot is across a 5K fix resistor. The pot has to go much-more than 5K to be "no effect". 22K is not really "much more". However the pot also has to go much-less than 5K to give "big effect". A 50K pot gets too twitchy in the range zero to 5K. 22K is a practical compromise.

This is not essential. The form with a 5K pot works fine, but the Presence never really goes away.

[Platefire]

Thanks for the explanation PRR. I really have a hard time really detecting the effect of a presence control---in other words, not very dynamic a change. Best I can tell the amp seems a hair brighter with more presence. I usually just set it half way and leave it at that. Maybe someone could tell me what to look for or I should expect for with that control---but to me it don't seem to have much of a effect.

On another subject---NFB---My speakers all have 8 Ohm loads, so I will be using that tap for a single speaker jack. To my understanding I need to use the 4 Ohm tap with the NFB. I really don't want to install a separate 4 Ohm jack because I'll never use it-----soooo how about I run the OT 4 Ohm tap up on the board and solder it to the NFB resistor turret??? I would like that better than installing a 4 Ohm jack just to run NFB to---Huh? Platefire 

BTW- I cut and pasted this information on the Presence control of Marshall Plexi History:

Earlier Marshall amps had a 5k Presence Pot with no Presence Resistor in parallel. The Presence Pot was changed to 22k and a parallel 4.7k Presence Resistor was added  to eliminate pot scratchiness due to DC voltage across the pot. In the new circuit (shown above) the Presence Cap blocks DC and keeps it off the pot. The 4.7k Presence Resistor also backs up the Presence Pot in that if the pot fails the amp will continue to function. In the old circuit if the pot fails the amplifier will completely cease to function.

[sluckey]

On another subject---NFB---My speakers all have 8 Ohm loads, so I will be using that tap for a single speaker jack. To my understanding I need to use the 4 Ohm tap with the NFB. I really don't want to install a separate 4 Ohm jack because I'll never use it-----soooo how about I run the OT 4 Ohm tap up on the board and solder it to the NFB resistor turret??? I would like that better than installing a 4 Ohm jack just to run NFB to---Huh? Platefire 

You just asked this very same question 2 days ago. I answered then you deleted the question.

Now who's having a senior moment. 

[Platefire]

Well this is not actually a senior moment, even though they are plentiful these days. It's more a case
of probably unfounded "Taboo" perceived amp practices. I did size up the situation of installing the second 4 Ohm jack, but I didn't like it because it would put the jack right above the PI tube socket---"whoa Taboo". So the perceived Taboo is--- I've never put a speaker jack right over a pre-tube in that close proximity before--so the reason of my hesitation! See picture. Platefire

[sluckey]

Don't think of the PI tube as a preamp tube. The PI is actually part of the power amp. If you look at the schematic you'll see that the NFB wire from the OT secondary actually connects to the PI tube thru a couple resistors. My point, it's perfectly fine to put that other speaker jack near the PI tube. I would.

IMO having a 4Ω jack and an 8Ω jack available is desirable. Just don't use the switch lugs on either jack as mentioned earlier.

[Platefire]

OK, Thanks! I'll put her in there. Platefire

BTW-I needed some knobs and heater wire for this amp, so I went ahead and ordered a 25KL from Doug
for the Presence pot. I suppose a silly 3K won't train-wreck the operation:>)

[Platefire]

I thought I would post the schematic of Doug's 50 Watt Plexi with Hot Switch which is what my design is based on. The fact that I am using a Power Transformer and Choke out of a Hammond PR-40 Organ Tone Cabinet puts a little bid different twist on the power supply. DummyLoad helped much with the original design including the drawings/schematics/Test on his PR-40 power supply to provide help/design of the power supply. DummyLoad is no longer hanging around here so I would like to say a big old Thanks! and hope he's doing well. If you run though all the post, many have contributed along the way--thanks! Sluckey has also helped me much along the way and still is, I appreciate that much. I am getting close to the end and expecting good results---but there will always be some corrective tweaks! So for anyone following this project I think it's important to see Doug's schematic compared to mine. So Doug's schematic is first and then mine. Platefire

[Platefire]

Question? Getting ready to wire up V2 Socket. Regarding V1a I have a plate resistor on my schematic of 330K. Researching this I find the model 2204 schematic has the 330K and the model 1987 has a 100K plate resistor. Also Doug's 50 watt Plexi with hot switch that my design is based on has a 100K in V2a.
So I'm inclined to think it should be a 100K in that position but my question is---why did they use the 330K on the 2204 to 74VDC---more distortion? Would kind of like to understand it before I install. Platefire

BTW-If you would like to view a 1987 and 2204 schematic, they are attached--
 

[PRR]

> why did they use the 330K

An obvious answer is: less heater-cathode stress.

As there is another stage of gain before the power bottles, this is still not the primary distortion point.

I doubt it is a huge difference, but you could compromise on 180K.

[Platefire]

Thanks PRR, I went ahead and dropped a 100K in there to be consistent with Doug's. It can always be changed latter if it needs to be. There usually always is some tweaking on a new amp anyway.  Platefire

BTW-----All done except heater wiring

[Platefire]

Fired up the Hot Shot 48 first time Wednesday about 1:00 PM. Tried it on a light bulb limiter first without tubes, then with tubes and everything seemed good so went  ahead with full fledged voltage. Everything seemed to be working well. Clean channel without hot switch worked well and even sounded kind of fendery to me. The hot switch was some fine sounding distortion. Very thick, rich and very Marshally sounding I would say. The amp seem just a little noisy with hum to me especially when the hot switch kicked in.

I did a complete voltage check and adjusted bias. The power tubes were a new matched set of EH EL34's. Not very well matched IMHO at 8mA apart. At 385V plate voltage I set to balance between the two at 35.3 mA/43.1 mA

Before I had a chance to test is very much, I had a major accident. I was re-checking the plate voltage on V5 EL34 and my MM probe slipped touched the socket heater terminal at the same time and flashed brightly:>/----then no more sound. The PT was still working but no sound. All of a sudden checking voltage there was a 624VDC coming off choke. I figured I had taken out the choke or the OT or both.
I was so disgusted, I just put everything up kind of sick at heart. So I pouted the rest of the day in disbelief that I had done such a stupid thing---so close then blew it.

Well today Thursday I finally recouped myself well enough to check it out. Still building up to 624VDC after the choke but nothing on the EL34 Plates. After messing around with it for a while trying to figure out what was going on, I finally put a jumper cable over the first 5Watt 100 Ohm resistor---Bingo---the amp came back to life--plate voltage on EL34's normal. So instead of taking a major item out is was just the first power resister after choke. All the voltage from the power supply was accumulating on the first filter cap and was stopped on that open resistor. What a relief--happy again!

Attached is the schematic with all the voltages readings and all look good to me with the exception of V2b at 197V and the D rail shows 274V. With no resistor between that, should be 274V? I'll check that out further plus the hum issue when I get my resistor in, none that value in stock.

Looking forward to getting a new power resistor in to play around with it some more. Just thankful that a resistor is all that got damaged in the accident. Platefire

[sluckey]

V2b at 197V and the D rail shows 274V. With no resistor between that, should be 274V?

Correct. You should recheck that. Maybe just an operator error.

I would completely remove R32 and C16A.

[Platefire]

Apparently you think R32 and C16a is overkill? Not sure why Dummyload designed it that way. I know
on the PR-40 power supply he tested with diodes I recall him saying the voltage spiked up real high for a few seconds before coming down to working voltage---may not be the reason at all? For sure that's why he recommended the 600V caps.

So do you think those two components could be having a negative effect on the amp? Thanks, Platefire
 

[sluckey]

Not a negative effect. Just not necessary. DL was thinking you may need to drop the B+ a bit but that's not the case. Those EL34s will be happy with 400v or even higher. The 100Ω will cause the B+ to sag depending on load. That may or may not be desirable.

You've seen how high that B+ will go when that resistor burned open. No current flow (load) causes the swinging choke to not do it's thing and the B+ will rise to it's maximum amount.

[Platefire]

Thanks, sluckey. I'm going to leave it as is for now. If it gets to loose and flabby, I'll remove it latter to try to tighten it up.

i got a 5 watt 15 Ohm from a amp tech friend in the next town and put that in there today. I think is sounds better than my Dukane Bassman. It's just seems to have more depth or thickness. The wife and I ran through some of our JamMan songs using my Hwy 1 Tele and it made the tele sound big. Takes pedals well. My experience has been very good so far. It was operating very quiet at the volume I was running it tonight. 

I did double check that V2b plate voltage and it was same as the D rail as it should be. The voltages all picked up a volt or two with the lesser Ohm power rail resistor. . Platefire

[sluckey]

You did a good job with that. I bet it becomes one of your most favorite amps. The transformers look like little soldiers standing at attention, just waiting to follow you anywhere.

[Willabe]

Those are some big soldiers! 

[Platefire]

Forward March! Hulp two three four>>>>>>>>Charge---->------>

As I recall those solders are standing up erect and at attention because sluckey told me about "L" brackets I could get to mount them this way. Up to that point I was scratching my head how to mount in a used chassis already full of holes. Think I'll name that PT "Mad Dog" after the new appointed General%>/

Something I played with this morning---being the type player who likes the old bluesman's approach to using my guitar volume to nudge cranked tube amps to go from clean to dirt, with the hot switch engaged and the pre and master vol's at low settings, you can go between clean to scream pretty successfully with a little practice managing it. Makes me kind of interested in hearing how Doug came up with that design. Platefire

[frankenxtein]

It's very nice , the cap$ , OT , tubes , I'll say there is a power supply available here in Memphis until I get enough nerve$ to build with it. 
Very nice , thank you for sharing your design.

[Platefire]

Your Welcome. All I can say is these folks at this forum has been helping me through builds for 15 years and they are the most friendly and helpful folks on the planet.

I bought a piece of wood today at LOWES and going to start on a head cab for it. Right now the bottom of the chassis had no cover plate and it's just open. So I need to do something about that. Platefire

[sluckey]

Right now the bottom of the chassis had no cover plate and it's just open. So I need to do something about that. Platefire

Put the bottom shield in the cab. Just glue/staple some heavy duty aluminum foil to the bottom of the head cab. Or use some metallic screen wire. Or my favorite, 3"wide aluminum duct tape.

[Platefire]

That had crossed my mind, if I needed to do that? It is on every commercial amp I'm sure for good reason. Thanks, I'll just have to decide what I will use. I kind of like the idea of the duct tape but just wonder with it sitting under the chassis, can't help but wonder if the tape would tend to pull up with weight of the chassis sliding across it when taking the chassis in and out. I might need to find something tougher because the chassis is really heavy with those big transformers. Platefire

[PRR]

> if I needed to do that?

You don't need it until the day you NEED it. Working a club or garage with ugly electricity. Working too close to a broadcast transmitter, or the CB and Police radios at the truckstop tavern.

I've been embarrassed often enough (polka-hour radio in the harpsichord recording) to want to do all the "easy" things before Murphy can find my build.

[Platefire]

Hay PRR, definitely don't want no Polka creeping it there  or 10-4 good buddy!

[Platefire]

Worked on this amp several hours today. First it was the noise problem then second was the unbalanced power tubes about 7 to 8 mA differance.

I spent a lot of time checking lead dress, grounds and looking for some kind of short but never ground nothing and the noise didn't change. Even put the bottom board of my head cab over the bottom opening side of the chassis with a piece of tin foil but still no change. I finally gave up on the noise issue and moved to the biasing.

I had recently bought an extra new single EH EL34 to hope I could somehow match it with one of the existing so called matched set to hopefully get a better match. So I checked the existing tubes first at 385 VDC/38.3 mA abd 385 VDC/46 mA.

So I pulled the one running 38.3 and put the new tube in that slot at 45.2 mA/49.3 mA---that's a little closer but a little too hot.

Checked the voltage and both plate voltages had gone up to 390/390. So I reset the bias 40.7/39.6---not hot but not cold either and very well balanced.

Ok so I'm done, sitting my amp back on my speaker and it hit me, try a different preamp in V1, 'V2 and V3 that seemed to be kind of useless because they are all suppose to be new. Well I started with a known good replacement with V1----Wow somethings different, all of a sudden I begin to realize most of my noise is gone>>>>I just would of never suspected that. So I had all new EH tubes in it but now one used Groove Tube in V1 that sounds real good. Still a little noise but not nothing as much as before. So I'm very thankful for a little success balanced power tubes and and a quieter amp Ye haw! Platefire

[PRR]

8mA unbalance is totally fine for EL34, which will typically perk-up to 250mA when actually playing big.

You do not want either tube over-heating, or sitting dead. But a 50:30mA unbalance may have almost no effect on "sound".

> they are all suppose to be new.

I pulled a new holiday-light string out of the box. 1/4 of the run was dark. Tubes are not made a whole lot better than holiday lights. New does not prove good.

[Platefire]

Yeah I know that close of a match is overkill but it's a nice Christmas present to have my new amp finally whole and healthy. Thank you Lord! Platefire

[Platefire]

Got an issue with this one I've been noticing lately. A frying noise that I assumed was pre tube messing up. I subbed V1, 2 and 3 with a known good tube with no change. I got to playing around with the controls and it seems no other control effects the volume of the frying like the treble control.
Turn it down and almost non existent and turn it up and it gets louder.

So the first thought that came to mind was the 500p Silver Mica Treble cap but I don't ever recall one behaving like that.

I haven't tore it down yet to check anything internal. Anyone got any ideas on this? Platefire

[sluckey]

I assume turning the master to zero kills the frying noise? If you turn off the hotswitch does the frying go away? Change the three 100K plate resistors in the preamp. Also change the 100K cathode resistor.

[Platefire]

Thanks Sluckey. The master kills it. The hot switch is noisy with white noise but no frying. Only frying with the hot switch off. Platefire

[PRR]

Is the SM cap good 500V rated?

Used to be they all were. In recent years you can buy 50V rated. With the variations of natural Mica, this might stand 250V for a while then start to break-down.

I think for the bench-cost of 500p, replacement beats thinking.

Or just remove it for a moment. Sound will be dull, but if the frying stops that's some confirmation the cap is sick.

[Platefire]

I did a little more testing. Put it on standby(SB), pulled V1, turned the SB off was still frying. Put it on standby again and pulled V2 and turned SB off----no more frying. So what does that prove?? nothing related to V1? Not power tubes with MV down no fry. So that puts it at V2 or V3 and since the treble control severely effects it, that would put it down to V2 and tone stack at most likely culprits.

Am I thinking right? So the V2a 100k Plate R mounted on V2 socket, the V2b 100K cathode R and the 470p treble cap would be the most suspect in that area? Platefire

BTW--on the SM, I really don't know for sure if it's rated a 500V---it should be! but I couldn't say for sure until I take the bottom plate off and have a look.

[silverfox]

Could this be oscillations?

silverfox.

[Platefire]

It don't act or sound like oscillations. It's just a low level static, mild popping, not loud at all but just enough for me to know it shouldn't be there and bug me. Turn the treble to 9:00 UN-audible, 12:00 barely audible, at 3:00 audible. I just got through playing about an hour on it and it sounds  and operates great, just need to stifle that little gremlin. PLatefire 

[PRR]

I think you want to pull the cover and replace that treble cap.

For motivation: if that cap goes from leaky to dead short, it wants to put big DC on the next grid, which is not a good thing.

[Platefire]

Yes, Thanks! I really shouldn't run it anymore with this possibility looming.
I checked my inventory and couldn't find one of the brown SM caps like Doug sells, but found
A black one I think is NOS. This is 500p isn't it? Pretty sure it is, please verify.
Platefire

[shooter]

This is 500p isn't it

Yup, believe also Doug sells those, or did

[Platefire]

The SM 500p was the culprit. It was rated at 500v too. Thanks so much for conferring with me to find it. It's quite as a church mouse now and I'm a happy camper now. Let the Jamming continue.
  Platefire

[shooter]

SM 500p was the culprit

FWIW
That 500 in your pic, I've had a very high fail rate, like 1 in 3. bought over time. 

[Platefire]

Are you referring to the black SM or just SM in general. I've always had the brown SM and this is the first one I had failed in 15 years. I think I got the black one here in some NOS caps I bought about 10 years ago at a thrift shop. Platefire

[shooter]

The black one's, I've got them isolated so I don't grab by mistake

[Platefire]

OK, Thanks for the warning. Next time I order from Doug I'll get several brown SM 500p's, seems like they are always in demand. Platefire

[tubenit]

Platefire,

Great job on the amp!  And THANK you for sharing the schematic with voltages!   I put your SCH schematic in the SCH Library of schematics.  IF you have a layout to share that goes with the schematic, can you please add it also.

The link to your schematic is here:  http://el34world.com/Forum/index.php?topic=21357.0

With respect, Tubenit

[Platefire]

Thanks tubenit

I don't really have a layout for the whole amp. I started one using ExpressPCB as a format but there was so many amendments to the plan along the way I never brought that layout up to current.  It's a real headache using that program for a layout IMHO. I might try to resurrect that old layout and see what I can do with it.

I do have a layout for the board layout that Dummyload created for me but it has a few mistakes on it that I can't correct because I don't have the drawing program he has. Here is the last version he provided. Platefire

[silverfox]

If anyone is interested, I had another post regarding my find at GoodWill warehouse sales. A PR-40 cabinet. I've got one of those power supplies sitting here and don't expect I'll ever use it. Let me know if there is any interest.

silverfox.

[Platefire]

Yes, it seems kind of appealing but the shipping would be major
On that heavy buddy! Talking about a boat anchor

[Platefire]

Making progress on the head cab. A few pics! Front and rear panels coming.
Thinking of finishing it with black tolex. Platefire