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

[Platefire]

I've got an old abandoned project that includes chassis, populated board less transformers. Also got a board layout sketch showing component connections. It is a Marshall JCM800 pre-amp into a AB763 Fender Twin PI, PPIMV, Power amp and power supply.

So I'm counting the cost rather I want to resurrect the project or not. Looks like I will need a PT, choke and OT mainly.

So my question is, I have a huge Hammond PR-40 Organ Tone Cab power transformer and choke. Doug's Twin PT shows 345VAC on each red secondary. The Hammond schematic shows 415VAC on each of its red secondaries that goes to two 5U4GB's. So that appears to be 60VAC too much more than needed. So if I used the Hammond, I would be hooking the two red secondaries to the AB763 diodes arrangement on the board. I just need someone to take a look and verify that there is no work around to make it work??? Attached is the PR-40 schematic and the PT I have is shown in the upper left hand corner. Also I have attached a Twin layout that shows the diode arrangement that's on my existing board the Hammond PT would be hooked up to. Thanks, Platefire   

[sluckey]

There's usually a workaround. You can use the Hammond PT ***IF*** you use a choke input filter just like you see on the PR-40 schematic. That choke has to handle the current for the entire amp, so you can't use the typical 'twin' choke. If you put that Hammond PT in a regular AB763 power supply your B+ will be about 586VDC!

Also, the Hammond PT has no bias tap. That's OK. You just use one of the red wires to also feed the bias rectifier. Replace that 470Ω/1W resistor with a 100K/3W (actual value may even be up to 220K to give the correct negative bias voltage to the bias pot).

So, if cost is a factor, go for it. You'll need a different choke, but it should not be expensive like a twin PT. You may have to study some choke data sheets to find a suitable choke. It would be nice if you have the choke from the PR-40.

PS... That Hammond AO-63 conversion I just did had a PT with high voltage. It used a choke input power supply so I just kept that part of the circuit.

[Platefire]

Thanks sluckey

I do have the choke. So that's making the project maybe have a little more potential. The choke is large enough in size where it would appear to be the size of an OT on a lot of our guitar amps. The PR-40 amp had a separate chassis for the power supply with the female 6 pin input connector from the organ/preamp, output to the power amps and the two 5U4GB's.

I actually made a console connector to the power supply by using an old battery charger chassis and installed stereo input jacks to go to G1 and G2 grids, fuse and about 10'-0 of 6 wire cord with a male 6 pin Plug to plug into the power supply chassis female socket. I used a Art Command Center Preamp/Processor plugged into my battery charger interface and played it as a guitar amp several years. I had the big cab set up in my living room. Made a nice piece of furniture. I ended up getting rid of the cab to make room for other stuff and kept the amp/power supply in the storage shed. Thank you, Platefire

[PRR]

The Hammond sports three 17 Watt amps. Total 51 Watts out.

The proposed amp is four 6L6, potentially 80W-100W out.

Are we sure the current is there? Or will the 6L6es be run below their max output?

As a *two* 6L6 plan, with two-6L6 OT, I'm sure it is ample.

[DummyLoad]

in the stock config the 3 pairs of 6BQ5 + reverb driver are drawing ~297mA at idle [ 3 pairs 6BQ5 with Vk at 12V / 130R is ~92mA per pair + reverb driver 12BH7 with Vk at 12V / 560 is 21mA  ]. power transformer is choke loaded. with SS rectifiers and choke loaded we'd probably see around 375V loaded but with plate impedance of 5U4GB we lose another 50-60V and we end up with ~330V.


with CLC filter and SS rectifiers you'll see about 580V loaded. if you use the pair of 5U4GB then PT will push a pair of KT88 at around 540V B+ without much effort. stack 350V filters. use the PR40 choke. make first B+ tap for OT AFTER choke. so you'd have: RECT-C stack - PR40 Choke -C stack then OT tap: the rest of PS taps as usual. use uF5408 diodes with at least 2 in each string. for the OT use dynaco/sunn A431S in UL form & AB763 LTPI @ ~440V and definitely use NFB - copy the fender NFB and tweak to taste. bias supply will need to be HV tapped. bias at about -80V. output power should be about 100W.


another option: leave choke input and replace the 5U4GB rectifiers with SS rectifiers or 5AR4/GZ34 and you'll bump up B+ to 375V, if you use the 110V primary tap with 120V source then that bumps B+ up another 25V to ~400V or thereabouts: YAY! 400V! the down side is you also bump up heater string voltage around 10% & you'll probably need to burn some off. if you run with the 375V B+ then a tweed twin or tweed bandmaster ckt. with 2x 6L6x would be a fit or 5E3 type with 4x 6V6. use 4K super reverb OT with either. no bias supply needed with the tweeds.


if it were mine, i'd push a pair of KT88 without even a second thought.


--pete

[Platefire]

Thanks very much PRR and Dummyload for presenting possible working options.

Here is a pix of the old project. I been robbing parts off it when I was in a bind. I do have a pretty good inventory where I could put a lot back.

I see the next thing I have to do before anything else----see how close the PT hole is to end of board. I need to go ahead and pull the Hammond PT and choke and see how I can manage the real estate.

I'm not familiar with KT66 tubes. Why do you likum? Isn't that the tube that Hendrix preferred? Thanks Platefire

[PRR]

> familiar with KT66 tubes.

Where did that come from?

KT66 is a European 6L6, pretty near. Expressly made to replace US 6L6 in many ways, was for a time better than US 6L6 but now that all the old tooling is worn/lost, it's probably all off the same stampers.

DL mentioned KT88, which is a double-size 6L6. 6550 appeared so Ampex did not need thirty-two 6L6 in the first videotape machines, just 16 6550. KT88 is a nearly-same tube (usually an inch taller) in European numbers.

If you want a 100 Watt amp, a pair of 6550/KT88 is an option.

However DL's excellent rundown shows Hammond only expected 297mA at (SS) 375V, which is 111 Watts DC, and likely about 55 Watts Audio. Suggests ~~2.5K loading, which (if you can find a 2.5K 50W OT) does suggest four loafing 6L6 or two 6550/'88s. A 3.4K ("Marshall 50W") load suggests EL34 and about 40W output.

[Platefire]

Hay PRR

Yes KT88, don't know where I come up with KT66 other than I'm not use to dealing with those tubes. I've been looking at KT88 on e-bay---they are pretty proud of them

I'm not really interested in big output power. I was really wondering about the 4-6V6's and DL has covered that plus several other options. I have been reading it and trying to figure it out. PRR you right, DL did a great job of breaking down my options and man I appreciate that. That helps me a bunch!

Not saying that this is the way to go but let just explore the 4-6V6 option a little. DL, if I understood you right, you said use the Hammond choke, use the SS rectifiers for 400V B+. Is that loaded or unloaded? I assume unloaded. So I've never had to burn off heater voltage, would you do that with some resistors?? A super reverb 4K OT would work for that?

I haven't abandoned your KT88 suggestion, it's just they are kinda high and I'm a tight wad. On the other hand if you suggested them above everything else, it must be something to them. I imagine big clean power?? but only guessing. Everything I've built has been Fenerish and maybe it's time to go Marshallish.  The JCM800 preamp is what's on the preamp part of the board now---with my situation would I need to go two EL34's to do that on the back end? or would KT88 be even better?

This board sketch is basically what's on the board except what I've removed for parts. So this may give you an idea what's presently on the board. The sketch was 11" x 17" so I scanned it as two pdf files. Thanks, Platefire
 

[DummyLoad]

...explore the 4-6V6 option a little. A super reverb 4K OT would work for that?

yes. 4 x 6V6 -or- 2 x 6L6  - if you're pinching pennies you could sub the vibrolux OT: it's 4K 35watt, choose hammond 1750J for 4 ohm secondary or 1760J for 4-8-16 secondaries.

if I understood you right, you said use the Hammond choke, use the SS rectifiers for 400V B+. Is that loaded or unloaded? I assume unloaded. 

yes, use hammond PR40 choke

yes i'm assuming slightly over 400V at idle and loaded with 120VAC applied to the 110V tap. in this plan, you'd shave off a volt or thereabouts of filament supply voltage. use resistors calculate value with ohm's law.

with 120V to 120V primary & SSR you'll be close enough to 380V to build a tweed twin type 2 x 6L6G power amp or "super" tweed deluxe 4 x 6V6 amp.

test PT on your bench and work with care:
1) apply 120V to 110V primary tap. it's the pair of wires of the three primary wires that have the least DC resistance and record the DCR of 120V & 110V windings;
2) measure filament windings AC volts and record values for both of them  the 5V and the 6.3V;
3) measure HV winding AC volts: measure HV winding from CT to end and record: don't measure HV winding AC volts end to end as you may exceed your DMM's input rating;
4) measure the DC resistance of the HV winding end to end and CT to each end and record;

apply 120VAC to the 120V primary and repeat steps 1-3.

--pete

[Ed_Chambley]

Plate, sorry for jumping in so late.  I built an amp using one of these and used an input choke.  I used 4, 6v6 tubes and an AB763 with verb.  I have to say it plays cleaner than any other amp except a Standel and I would bet if I changed the PI to a 12aU7 and used a JBL it would be just as clean.

I was very happy, but it was too heavy.  It sold the same day I took it to the music store.  Did I say I put it in a 2, 12 Cab with 2 vintage 30's.  Thing weighed in at a whopping 88lbs and I did not put casters on the combo.

If I may make a suggestion tho.  If I were to do it again and I do have another one, I would just use 2 KT88's.  I have built 2 KT88 amps now and absolutely love them.  I also have plenty of 6550 Winged C and some original TungSols as I have repaired Leslies for a while now.  They have a very unique tone for guitar and you will have something different.  There is absolutely no mud when using these tubes, that is unless you want it.

The tone and frequency response is better than a Twin IMO.

Good luck with you build, just thought I would chime in and say HI.

[Platefire]

DL, got the PR-40 Power supply chassis out this morning plus also my PR-40 Guitar Preamp/fused power supply Interface that I made years ago to hook guitars up to PR-40's through 6 pin connector that came from the organ(but now my interface) to the PR-40 power supply chassis. So I got it hooked up and it's working.

I am trying to perform the test you outlined as follows:

1-I'm still a little confused on this one. On the PR-40 schematic, it shows the two red primary taps that the hot and neutral 120VAC hook to. Also on the sche shows the center tap labeled "HO". So if I'm understanding you right, you want me to measure DC resistance while it's hot! I'm not understanding--" measuring the DC resistance of the 120 & 110 wingdings". Please elaborate on that a bit more---with it being measure hot, I don't want to hook up wrong and blow something up. 

2-Measurements on the two yellow wires is 5.39VAC and the one connected to the choke is 6.10VAC. Heater voltage is 3.90VAC per side, 7.79 combined.

3-Little confused about this one to. When you say "HV" is that High Voltage? With the MM grounded to chassis and reading the secondary ACV is 455VAC per side. Also there is a tap called "-HV" I'm assuming to the bias circuit, it reads 50VAC.

4-Is this measurement to be done hot or not? Measured not hot with the rectifiers pulled, MM ground to chassis and MM red to rectifier pin #4=24.8 Ohms and to pin #6=27.5 Ohms. 52.4 Ohms end to end. If I did it wrong please tell what I need to do to correct.

BTW-I measured the unloaded B+ with both 5U4's in there and it was 473DCV. All the above measurements was made with everything wired up Hammond style it the chassis. If I need to cut the wires loose to the rectifiers/chassis to do the measurements, just let me know. Thanks, Platefire

[DummyLoad]

please measure DC resistance WITHOUT power. never try to measure resistance in powered up circuit. you can fry the DMM/meter.


--pete

[DummyLoad]

forgot to mention - remove the tubes and discharge caps.

seems that there are two versions - the PT you have doesn't have the tag-board on the end-bell. your PT may not have the 110V tap?


so you do have a 50V tap. if that's the case, then it can be used for 6L6 or 6V6 bias, however, it won't work for 6550/KT88, however, you state that you're leaning to 4 x 6V6 so that's a plus.


looking for pics i found an old ad on ebay for a pr40 amp. made him an offer, he accepted and so it's on the way. i want to experiment with a choke loaded supply and this one is perfect. the bounty of 6BQ5's is a bonus. thinking i'll convert mine to SS recto or GZ34 and run 2 x 6L6 for starters, then experiment with 6550/KT88.


--pete 

[shooter]

it won't work for 6550/KT88

can you help me with this?  Is 50vAC, to low for fixed bias 88's?

[DummyLoad]

it won't work for 6550/KT88

can you help me with this?  Is 50vAC, to low for fixed bias 88's?

what plan? 100W? 50W? B+?

dopends on the mode. pentode mode, yes marginally and dependining on B+, triode mode of any use no, UL mode likely not.

for UL you really meed 60-70VAC winding. 100W UL spec calls for 550V and 80V g
bias.

you could use half wave doubler. i don't trust them. use an ancillary transformer for bias if 50V tap won't work.

--pete

[Platefire]

DL, I think everything I previously told you about the PT primary's was wrong!

I now see what you mean about the 110V primary tap and 120V tap. Because the schematic PT wiring is not color coded to match the PT wiring in the amp, its a little hard to figure out which is the 110 and the 120 tap. There are three primary taps, two black and one brown. One of the blacks go to a terminal strip that's not connected to anything. So this is the resistance reading between those taps:

The black and brown that's connected to 120V is 1.5 Ohms between them.

The Connected Brown to the unconnected Black is .3 Ohms between them.

The connected Black to the unconnected Black is 1.4 Ohms between them.

On the schematic it shows pin #3 of the 5 pin connector from the console connected to the 120V tap primary. The one Black on my amp is connected to to pin #3 also.

On the schematic it shows pin #4 of the 5 pin connector from the console connected to the tap on opposite end of the primary winding from the 120 and 110 taps. The brown wire on my amp is connected to #4.

I measured voltage between pin #3(Black Primary) and #4(brown Primary) of the five pin connector with it connected to AC house voltage and it was 120VAC between those two primary taps. So these two taps is what's connected to power.

So with the above info, if you will identify which is the 110V tap, I will apply voltage to it and make the readings you ask for in replay #8. So is the un-connected black the 110 tap?


Another thing regarding secondaries that was wrong is, there is no bias tap. All three PP EL-84 amps in the PR-40 is cathode biased.   

I just imagine you was confused after reading my last post for item #1 for your requested measurements and I hope this clears some things up. Thanks, Platefire

BTW-I'm starting to think about a 50 watt Marshall. The large PT is crowding the area where the last two octals would go, so two EL-34's in the middle. The bias voltage comes right off the secondary. Something like this:

[DummyLoad]

So is the un-connected black the 110 tap?

yes, i believe that it is. use a lamp limiter if you test with 110V tap. 

--pete

[Platefire]

Well thanks pete for your reply.

I was kind of hoping to do the 50 watt Marshall with two EL34's but as you indicated about the heater voltage already high at 7.79V and going higher with the 110 tap. Also the EL34 would require 400V loaded plate voltage and with the 120 tap would be running 370V to 380V would be low for the EL34. Regardless, I was planning to do SS/diode rectification on this one.

I did some measuring last night and checking my real estate. I'm not sure if I can use the two further most octal socket cut outs because the large PT will be right up against those tubes if used. It appears the tube will be within 1/2" of the PT. That might not be a big deal? but I've always tried steer clear of that. That's why I was thinking about the two middle octal slots for two EL34's or 6L6's.

So it appears the only way to get the loaded voltage up enough to do that is by using the 110 tap or a voltage doubler---right? So it will be Monday before I'll have opportunity to test it further. So if you think there is a work around for the higher voltage, I will test the 110 tap then and post the results.

If the 50 watt Marshall is out of sight, I'm sure I can juggle the PT around someway to get clearance to do the 4-6V6 thing. Thanks, Platefire

BTW-I can't believe you have picked up a PR-40amp. I've had two. Nice speakers, tubes, transformers and cab. Lots of parts to scavenge off of that! Do you recognize the part that I used to install a MV and hot switch in the pix?? 

[Platefire]

Ed

Just wanted you to know I haven't ignored your post, just been real busy
I might end up doing the 4-6V6 thing but for some reason I've got interested in a 50 watt Marshall. That's something completely different than I've ever done and I've never owned a Marshall amp--so this would give me a little taste.

Did you make a schematic of your AB763/Super 5E3 power amp? Did you fix bias or cathode bias it? One channel input? At this point I think my voltage levels will be to low for the two PP KT88's. You and DummyLoad both recommending them has got me wondering about a KT88 sound.

So I'm not sure which way the amp is going to go, but will definitely be going. No hurry, I love piddling with this stuff Platefire

[DummyLoad]

I did some measuring last night and checking my real estate. I'm not sure if I can use the two further most octal socket cut outs because the large PT will be right up against those tubes if used. It appears the tube will be within 1/2" of the PT. That might not be a big deal? but I've always tried steer clear of that. That's why I was thinking about the two middle octal slots for two EL34's or 6L6's.

use the two holes next to the PT for can caps. JJ or F&T 1-3/8" cans will fit nice in those holes.

--pete

[Platefire]

Yes, could do that. Think for now I will concentrate on getting the big PT and choke mounted. Then get power cord, power switch and fuse holder installed. At that point it might be a little more obvious with what room I have left, what to build in that space. After all the big Hammond choke belly will have to be cut into the chassis just like the PT. That may have an impact on how my board will be situated. May have to build part on board and part terminal strip P to P. Platefire

[sluckey]

You can easily put some small "L" brackets on that iron and stand it up to avoid cutting big holes. That will free up a lot of real estate under the hood. Does the PT and choke have bell covers on each side of the laminations?

[Platefire]

Well this is what it looks like on top and going through the PR-40 power supply chassis.

[sluckey]

I would get 8 L brackets and stand both of those up.

[Platefire]

So your saying to change the mounting style from flat to Horizontal? Who handles the L bracket footings?

Like this or that?

[sluckey]

Lowe's has a good assortment. 1/2 x 3/4 x 3/4 is what I've recently used for this same purpose. Should be able to find them in any hardware store, even Walmart...

http://www.lowes.com/ProductDisplay?partNumber=315714-1277-DPB115&langId=-1&storeId=10151&productId=3169855&catalogId=10051&cmRelshp=rel&rel=nofollow&cId=PDIO1

[Platefire]

Thanks sluckey for making me aware of that. Never done that before on all my conversions. Doing that may allow me to use the existing board made for that chassis with a few alterations for what ever circuit. Those huge things(PT/choke) will look kind of funny and top heavy jacked up so high, but sometimes extraordinary situations calls for extraordinary action. I think that will be next thing on my list to work with the intention of making it work. Not sure how it will work around that big gaping hole for the old PT, but I have built cover plates before for such holes. Get the L brackets, put them on and work it from there!!Whatever it takes,  git r dun     Platefire

[Ed_Chambley]

Ed

Just wanted you to know I haven't ignored your post, just been real busy
I might end up doing the 4-6V6 thing but for some reason I've got interested in a 50 watt Marshall. That's something completely different than I've ever done and I've never owned a Marshall amp--so this would give me a little taste.

Did you make a schematic of your AB763/Super 5E3 power amp? Did you fix bias or cathode bias it? One channel input? At this point I think my voltage levels will be to low for the two PP KT88's. You and DummyLoad both recommending them has got me wondering about a KT88 sound.

So I'm not sure which way the amp is going to go, but will definitely be going. No hurry, I love piddling with this stuff Platefire

Build your marshall if you don't have one.  Here is my weird methods of building today.  I mainly build power amps and separate preamps.  Then I always have schematics and I do mostly point builds.  Cathode bias for everything 30 watts.  On the KT88 I have done UL with fixed adjustable (Marshall type bias) and also cathode bias as in the Leslie 147 with 6550 tubes and I paired a set of KT120's in a build as well.

I now have quite a few Power Amps with different tubes and some different PI's.  I just sold off a herd of amps I built and don't want to get anymore unless I run into a preamp/power amp I really love.  I guess what I am saying is I have all the complete amps I want and just like building different stuff.

Unless I am working on something I plan to put into a head cabinet and pair up with a speaker cab, I do not draw it.  I am currently working on a 4 6V6 amp that I am drawing a schematic and layout for, but it has an overdrive, dual input tubes, verb and tremolo and this makes it where to me drawing it out is needed.  Most of the stuff I like, Sluckey has done something similar and I simply revise his Visio files for my needs.

A cranked KT88 is a wonderful thing if you can stand the volume.  Their distortion is so smooth you really don't know it is happening until you are wiggling your fingers and it just keeps on going and they tend not to squeal very easily.  I have quite a few Marshall variants and love them.  Jimbob is always chanting about the KT88 Major as well.  I am sure you are familiar with old Purple so you know the sound of the tube.

I also have a Hammond JR-20 which originally used 2 OT's. one for Treble and one for Bass.  I converted it to a dual OT reflected 10K to 8 ohms as I had 2 Stancor OT's and used the Choke input.  I have a 6G6B preamp build I put in front of it and that is what gave me the idea of my current project that I have been working on for over a year now.  I only have the schematic and I have not completed it yet, but I am close.  I really like the Iron from these things and the JR20 has an alnico 12 it uses for bass and the good ones were made by Hepner, but when you find the Hepner the 4 10" are Magnavox and are not guitar friendly, but the hepner is a great speaker.

[sluckey]

Mounting transformers like this will be plenty strong. Lot's of transformers mounted this way. If you get serious about this, consider making an oversize cover plate. That will allow you to move the PT and/or choke around to suit your layout. Kinda like this...

[Platefire]

Ed---I haven't heard anybody doing it that way, with separate preamps and power amps. One thing for sure, it would be easy that way matching up different preamps and power amp to see how they work together. From all you have built, what was your favorite preamps and power amps separately and which made the best match joined together? Tell you what, I'm really impressed with the Ampeg Gemini II pre I installed in my Bogen CHB-20A. That was a departure from my norm that sluckey introduced me to.

I've been mostly tweaking my old builds. I have a little bit more knowledge now, so I trying to apply that to even making my old builds better. I got several others that need help . I'm really happy with the improvements made on the last two I've re-worked. So that's encouraging.  I really don't need more amps or guitars either for that matter. I have however started this build out of spare parts. It's like they are calling my name--"put me to good use" so I guess i will.

Thanks for describing the KT88 operation. That must be a sweet sounding tube but sounds way louder that I could use. I think you lost me on "Jimbob and "old Purple"?? unless you are referring to old Deep Purple? May need to q me in

So you've got an old Hammond project going on for a year now. The treble and bass OT's remind me of my PR 40 amp that has two treble OT's and one Bass OT. I've been told they wouldn't make good guitar OT's--but can't remember why? So do you have a thread for this build that your getting close to completing? I can't imagine how you used two OT's not being stereo? I never heard of hepner speakers, so that's a new one on me. Thanks, Platefire
 

[Platefire]

Sluckey

Checked my metal plate inventory today and what I got is in the 22Ga area. Seems kinda thin for this application. Think I may need at least 32Ga/ 1/16" thick. About a 5" x 5 1/2" would do it. I will be going by LOWES tomorrow and I'll check plates and L brackets.

22Ga might be Ok after being bolted down and stiffened up but I would feel better with 32Ga. Platefire

[PRR]

32ga is thinner than 22ga.

Strange but true.

What it is: the gauge number is sorta the number of times it has been beaten-down. I'm not clear on sheet techniques but I know some about wire. Wire starts as 1/4" rod. The first pull through a smaller hole gives #1 gauge, next pull gives #2, etc etc. #40 (pickup wire) is a lot of slim-down pulls.

Sheet probably starts from 1/4" or similar plate. Sheet gauges are not the same as wire gauges, but not-so-different, and go in the same direction.

(For practical reasons, electric wire over about #10 is never solid, always "stranded" out of 7 or more smaller gauge wires.)

[Ed_Chambley]

Ed---I haven't heard anybody doing it that way, with separate preamps and power amps. One thing for sure, it would be easy that way matching up different preamps and power amp to see how they work together. From all you have built, what was your favorite preamps and power amps separately and which made the best match joined together? Tell you what, I'm really impressed with the Ampeg Gemini II pre I installed in my Bogen CHB-20A. That was a departure from my norm that sluckey introduced me to.

I've been mostly tweaking my old builds. I have a little bit more knowledge now, so I trying to apply that to even making my old builds better. I got several others that need help . I'm really happy with the improvements made on the last two I've re-worked. So that's encouraging.  I really don't need more amps or guitars either for that matter. I have however started this build out of spare parts. It's like they are calling my name--"put me to good use" so I guess i will.

Thanks for describing the KT88 operation. That must be a sweet sounding tube but sounds way louder that I could use. I think you lost me on "Jimbob and "old Purple"?? unless you are referring to old Deep Purple? May need to q me in

So you've got an old Hammond project going on for a year now. The treble and bass OT's remind me of my PR 40 amp that has two treble OT's and one Bass OT. I've been told they wouldn't make good guitar OT's--but can't remember why? So do you have a thread for this build that your getting close to completing? I can't imagine how you used two OT's not being stereo? I never heard of hepner speakers, so that's a new one on me. Thanks, Platefire

Yes, I am speaking of Deep Purple.  I really Ampeg Preamps TOO!  I actually have a B12 Bass preamp and it sounds great into a Push Pull 6v6.

It is not the hammond stuff I have been working on for a year.  I ran across a 62 Bassman  a couple of years ago and fell in love with the preamp, so I have to say I prefer plate driven side tone stack.  I have also been messing around with signal pentodes a lot.  I finished a mod this morning and cannot get the photos uploaded as they are on my phone and the forum will not accept a .jpeg, just ,jpg.  I will put them up later.

The amp I am working on is just like a 63 Vibroverb with an added EF86 overdrive, but the PA section is 4, 6v6.  Something about having 4 output tubes that to me enhance and give a fullness you cant get with 2 tubes.   I like the Brown tonestack simply because it sounds similar to boosting a Blackface.  It adds just a little hair and spank and I love the reaction of the treble.  I wish I could get some more of the real 350K/tapped pots, but I read on another forum where Tubeswell posted an alternative design and it works well.  Not the same, but close.

I have a 68 Super Lead 100 Watt of El34's and a 50 watt built exactly the same, but the clean tone is much better with the 100 watter.  I don't know what it is, but there is something to it.

This morning I changed a JTM45 to KT66, added another hole and have 2 inputs to 5879 tubes.  My OT is not the proper one so I am running the amp 4 ohms into an 8 ohm cab and reflected I have 6.8K, so I ordered an 8K multitap a little bit ago.  The amp sounds great.  I have a couple things left to do, but will do them when I get the new OT.  I was surprised that the volume is not unmanageable.

I have one of the 5879's on the bright channel bypassed with a 3.3uf/2k2 and it acts like an overdrive when jumped.  I have yet to finish tweaking, but it is a winner.  I want to cascade switchable, so I got some relays to figure out.

Even though I sold a lot of my amps I still have a lot.  I have built preamps for a Blackface with verb, a Gibson GA40 and a 5B3 all in the same chassis, one of those blank stout chassis.  It is according to what I am playing, but I have to say my favorite preamp is 6sc7 driving a Brownface 6G6b Tonestack. 

It is big, fat and hairy.  Gotta love anything that is described like that.    I get more touch with the Octal pre tubes seems like.

Sorry for so much writing, hard to narrow it down really.  I have found I prefer less gain in V1 for my normal fingerstyle.
 

[Platefire]

Your right PRR, don't know what I was thinking about---going the wrong direction. I use to be a Draftsman in a Trailer plant(Nabors Trailers) 30 years ago and knew all the gauge sizes in fractions of an inch and decimals(16Ga/1/16"/.0625). This proves I'm a little rusty.

I did go to LOWES and found a piece of 16Ga x 6" x 18"($7) that I think is just right. It is unfinished steel, so I will have cut it off, Smooth off cutting edge, prime and paint it or it will rust pretty quickly.

Also found the 1/2"wide s 3/4" x 3/4" "L" Brackets by Stanly and just sizing them up(eyeballing them) with the choke and transformer think they will work just fine. The corners are not rounded off, so I think I'll have to round off one inside corner on each one with a file--easy fix.

It will be a little work to get the plate cut out, finished, mounted and then re-drill to get PT/Choke mounted vertically but when that's done I should have a lot more room to do what I need inside. Thanks, Platefire 

[PRR]

> a Trailer plant (Nabors Trailers) 30 years ago

http://forum.aths.org/85866/Nabors-Trailers?PageIndex=2
http://forum.aths.org/Attachment15932.aspx

[Platefire]

Yeah PRR, that's it that you referenced. Spent about 14 years there. My specialty was Low beds with detachable goose-necks. I also did pole trailers, flat beds, refrigerated truck bodies and moving vans but low beds was my specialty. The ones that haul the big cats and such equipment. I quit there in 1981 and went to work for Federal Government/US Army and stayed there 29 years--so 43 working years. The old original plant closed and one of the salesmen obtained the rights to the name and started his own Nabors Trailers on I-49 about 20 miles away. He latter changed it to his own name "Blackman" trailers but I don't know if he's still in business??

Glad to be building tube amps with 16Ga patch plates now

[Platefire]

Had a great Blessed Christmas day, Children, Grandchildren, in-laws and a few outlaws! Cleaned up the mess today and after a little relaxing swung back into amp mode.

Put the "L" feet on the PT and choke. Kinda sliding them around on the chassis trying to get best mounting position. This is my first thought with choke over existing grommited choke wire hole and PT positioned for easy tie in of red secodaries to diodes on end of board. I mostly thinking this way due to the wire exit holes and diode rectifier. If I was doing tube rectificatuon, I would face the Bell wire outlet toward the tubes. This look OK? Platefire

[DummyLoad]

suggestion: rotate the either the choke or the PT 90 degrees, that way the flux in the PT winding window won't couple with the flux in the winding window of the choke.


--pete

[DummyLoad]

my PR40 arrived this morning and it's in really fine condition. the amp & PS came with all the tubes.

first experiment:
no significant mods, other than the male 5 AC input was cut out and an IEC with fuse-holder installed. configuration is a stock choke loaded PS with the stock 50uF filter. i built a SSR octal socket adapter with 4 1N5408 diodes (see pics) and built a 4K 80W load bank. i estimated about 400V: real world is 386V. so 386V/4K is about 97mA or about 37.3 watt load. yes, the resistors got little warm: IR thermometer indicated that they were at about 135degF.

i ran a HV winding tap to one of the terminals strips to allow for a bias supply experiment as well. i'll post results of that tomorrow.

https://www.dropbox.com/sh/gw86gzvyacjujf5/AAAXWLa9ZAzU2bIAg-mWiCaZa?dl=0

--pete

[DummyLoad]

perhaps this is the ticket... makes sense. EL84 >> EL34.


--pete

[sluckey]

I really like that old Hammond stuff. Always seems to be high quality stuff put together very well. I'm waiting for some unmolested pics of the amp.

[sluckey]

Platefire, I would hold off on drilling any mounting holes for the PT and choke until I could put an OT on the chassis too. Then you can move them all around to see how they will fit. Sometimes you will find the OT mounted beside the PT like this...



Other times you may find the choke mounted beside the PT like this...



I would probably drill a hole in the bell cover for that choke to allow the wires to exit on the bottom (just like the PT). Completely remove the bell cover to do any drilling!

[DummyLoad]

I really like that old Hammond stuff. Always seems to be high quality stuff put together very well. I'm waiting for some unmolested pics of the amp.

link to some hi-res. shots of the amp chassis.

https://www.dropbox.com/sh/b04p7trqpjke5n0/AACE7QXji5rTcc2LgkAEucrza?dl=0

think, i'll leave this one alone. the power mod is fair game, but the chassis is in such good condition it'd be criminal to dismantle. i have the original sylvania and RCA tubes that came with it stashed as well. saving it for a PR40 cabinet that will need the service.

--pete

[sluckey]

think, i'll leave this one alone

I don't blame you. That does look nice!

[DummyLoad]

ok last of the experiments for a while. i need to get back to amp building. i need to finish one for a friend who also works on my M/C.


this looks good for a pair of EL34 in PP with a 3.5K load. use the marshall 3.6K from classictone or the marshall jcm800 50W from doug at 3.7K for slightly less power. datasheets say 48W with a 3.5K load with 375V B+.


the HV tap bias supply attached as shown has a span of -27V to -44V. to the B+ supply i added a 100uF dual can and another RC filter stage.


please see attached schematic for clarification.


--pete

[DummyLoad]

link to pics of experiment.

https://www.dropbox.com/sh/39jd7u0vjyj0jhz/AADuoceUrd5S7MVStwpgiHLUa?dl=0

one pic is bias pot at ~~ center, the other two are with bias pot at maximum and minimum.

--pete

[Platefire]

Wow! Things have been happening while I was out Trying to take it all in pete, the PR-40 you got in there looks like it just came out of the factory--clean. Both of them I got came from where I use to work, Fort Polk, La. And I happen to know they sat in an abandoned WW2 Chapel for years before the government decided to dispose of them--a lot of dust collecting! When they finally tore down the buildings, they had to do something with them then.

I'm totally blown away with how fast you moved out on this amp, modded the power supply and did a simulated load test, hooked up a test bias circuit and produced a schematic. How cool is that? beyond comprehension! Thanks! According to the way I'm seeing your experiment/calculations, everything it clear for a PP EL34 amp. So looks like a 50 watt(almost 50) Marshall is a go. So like you say, I need to choose an OT 3.6K to 3.7K with multi-tap Ohm. All of my speaker loads are 8 Ohm. I'll check out the sources you said.

sluckey, don't worry, I don't want to get ahead of myself of this one. I'm holding off doing much more until I can decide what Marshall circuit and get a plan together. Don't know much about which is good or bad regarding Marshalls. I know there is a lot of talk about a Plexi. I've been looking at the Marshall pre's with the low/high gain inputs and the way they do it through the input jack switch. Seems like to me it would be better to use a switch(or foot switch) to add the gain stage and stay plugged in. Also with 50 watts I think I need a MV and thinking post PI MV with dual 1 meg pot. Like you say, also need to get my OT on board in the planning stage. I have schematics on the plexi and 2204 looking them over. Would appreciate advice about the right Marshall circuit. Thanks, Platefire

[birt]

how about JTM 45 with EL34 (JTM50)?
http://www.classictubeamps.com/schematics/Marshall/JTM50.pdf

[DummyLoad]

plate, there is something else to consider before you start building: when you fire up a power supply with choke loaded PT with no load, the B+ shoots up to near that of a capacitor loaded PT.

with the tube rectifiers replaced with SS, the B+ shoots up to over 540V until there there is a respectable load - IOW, you have 540+ volts of B+ until the tube heaters warm-up and provide the load. with tube rectifiers, the power amp heaters have already started warming up when the rectifier tubes are warming up, so the overshoot is near non-existent.

IF you use 450V or 500V caps, you risk damaging them unless you have permanent dummy load built into the PS to keep the B+ overshoot in check until the heaters warm-up. there are other work-arounds: use 600V caps, or a series cap stack(s), or use a pair of 5AR4 tube rectifiers with slightly less B+.

do not use a standby switch if you use 5AR4 tube rectifiers with 450V/500V caps.

--pete

[DummyLoad]

plate, i ran my breadboard up with this PS just little while ago. i have a 4K fender OT connected. with bias at -32V i get 375V B+ with about 36mA flowing per EL34. sounds decent enough for 44W. the cut sheet says 44W with a 3.5K primary. it's loud, and it sounds fair enough, however, i'm driving it with a standel 25L15 preamp that was driving 4 x EL34 at 100W for maximum headroom. i was tearing the standel experiment down anyway so swapping in the PS mod and OT was a minimal effort. 

to alleviate the high B+ power-on surge issue i installed a pair of NOS sylvania 5AR4/GZ34 rectifier tubes. seems happy and it's relatively quiet even with all the damn wire running all over the place. surge at turn on peaks at about 415V as observed with my DMM. you could probably get away with one GZ34 with a loss of just a few volts of B+

please see attached schematics.

link to pics of experiment below:

https://www.dropbox.com/sh/39jd7u0vjyj0jhz/AADuoceUrd5S7MVStwpgiHLUa?dl=0

--pete