Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on January 24, 2019, 02:24:39 am
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Because I'm stuck on Fender amps I'm going to build a Princeton Reverb type amp.
I play clean mostly. I try other amps and combinations and keep going back to Fender amps so i think that's it for me.
I plan to use this amp as a clean around the apartment amp or for the quiet rehearsals I have with my band (almost acoustic)
I have an old Bogen VP20 chassis and transformers, mostly gutted, just some chassis mods still needed.
Basically a nuke and pave, using the transformers
Going to orientate it tweed style with pots and tube sockets on the edges.
Originally EZ81, pair of EL84s, and 3 12AX7 preamp tubes.
A battered but cool light plywood cab from an old regal amp, that the bogen chassis will mount in nicely
Just the right size and can accommodate a 12" speaker which is probably what I'll use.
a JBL D123 or another Rola 12" I have
I picked up most of the components today and had some left over from previous builds
I have a couple of typically ignorant questions:
- the Cathodyne PI Is it it any different sounding when the amp is used clean?
- Could I put a long tailed pair in this amp without adding a another preamp tube, say use the extra stage after the reverb? Any point?
-Which also begs the question could you build a single channel deluxe without the extra gain stage after the reverb?
- Has anyone built a Princeton with EL84 power tubes?
Though this PT seems to produce 420VDC at the first filter cap (.47) after the EZ81 rectifier (I'd probably use a solid state rectifier though) so maybe too much voltage for EL84s
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IF you are wanting a clean Fenderish PR sound, I would NOT use EL84 tubes especially with 420v on them.
Totally agree. Ream those bq5 holes to fit 6V6 sockets.
edit... Another idea. Your chassis is plenty big to add one more tube socket. That would allow for a two tube reverb like Hoffman's PR without tremolo. Or you could convert to a LTP PI, which basically makes the PR a single channel DR, like my TDR.
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Hey thanks a lot for the prompt responses.
I was initially contemplating the EL84s but the voltage the PT puts out put me off, still I was curious if it had been done. I bought a pair of octal sockets for the power amp.
I don't necessarily need this amp to be as loud, clean, as a DR, though it wouldn't be bad
It's for quieter applications
Also I was intending to make a standard Fender two tube reverb circuit. (though the one tube version is tempting too)
So I was wondering:
I know that in the PR and the DR there ends up being and extra triode
And as a result both have an extra gain stage right before their PIs
Can you have a PR (or even a DR for that matter) without that extra gain stage?
I couldn't find any information on this online so there must be a reason why nobody does it.
Is it too weak?
For instance with the PR/DR if you used a LTP PI using the extra gain stage triode
like this:
2 triodes - preamp and EQ
3 triodes - reverb
1 triode - tremelo
2 triodes LTP PI
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For instance with the PR/DR if you used a LTP PI using the extra gain stage triode
like this:
2 triodes - preamp and EQ
3 triodes - reverb
1 triode - tremelo
2 triodes LTP PI
Someone just recently modified a Hoffman PR to use a LTP PI. I modified the Hoffman board for him. Look back through the posts to find the thread. Probably won't have to go back past December to find it.
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For fabulous clean tone, I think any Blackface Fender circuit is fine. For overdrive, I prefer a LTP PI.
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For instance with the PR/DR if you used a LTP PI using the extra gain stage triode
like this:
2 triodes - preamp and EQ
3 triodes - reverb
1 triode - tremelo
2 triodes LTP PI
Someone just recently modified a Hoffman PR to use a LTP PI. I modified the Hoffman board for him. Look back through the posts to find the thread. Probably won't have to go back past December to find it.
Is it in this section? I was looking but couldn't find it.
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Guess you didn't look very hard.
Subject is...
Slucky/Hoffman PR w/o Tremolo, with MID, MV, and LTP build
Last post was by CordovaNate on Jan 18.
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I guess you're right
I was just searching Princeton with no luck and scrolling down through the listing intially i didn't see the 'PR' in that thread. I suspect my glasses prescription has changed again.
I was reading the thread and it has a link with takes me to another thread that's about a PR that incorporates a long tailed PI by doing away with the tremelo, thus using it's triode section.
I want tremelo in my amp
I was asking if I could do away with the gain stage/triode after the reverb but before the PI and use that to make my long-tailed pair.
like this:
2 triodes - preamp and EQ
3 triodes - reverb
2 triodes -LTP PI
1 triode - tremelo
As I said I can't find anything like this on the internet (and I only searched a bit) I know the non reverb version of the PR or DR doesn't have the extra gainstage.
Does the reverb version HAVE to have the the extra gainstage ?
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Sorry, I overlooked the tremolo requirement.
like this:
2 triodes - preamp and EQ
3 triodes - reverb
2 triodes -LTP PI
1 triode - tremelo
If you want a lush Fendery sounding reverb then you really need that third triode in the preamp. It's really a mixer for the wet and dry signals. If you can settle for a more subtle reverb, then consider Tubenit's one tube reverb. Then you could have your tremolo too.
Are you open to using a MOSFET for the tremolo oscillator, then you could build this PR with LTP PI (it's really just a single channel DR with little transformers). The MOSFET would plug into the exact same circuit as used in the PR trem. Easy to do.
https://el34world.com/Forum/index.php?action=dlattach;topic=24037.0;attach=74263
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I don't mind the idea of a mosfet tremelo.
The iron on this little amp is pretty good. Just judging by dimensions and comparing with the equivalents Hammond has on their site. The PT is about PR sized if not a tiny bit bigger but smaller than a DR
THe PT is pretty well the same size as a DR but standing on end.
A couple of curious things that I'm not sure about because I've had the chassis for a million years and gutted it without labelling EVERYTHING.
The OT has two taps for 8 and 16 ohms (green and yellow)and an extra secondary tap (orange)which is confusing
I was running 10 volts through the secondary taps to see how it was reflected in the primary to determine the turns ratio and thus the taps. I tried it with the orange tap and there was an ominous buzzing. So immediately stopped
The PT has a red and yellow centertap which was connected to ground. And another green and yellow tap which seems to produce approx 120 VAC
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The Orange for the past few PTs I've used was a ground for the copper shielding on the outside of the transformer.
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The OT has two taps for 8 and 16 ohms (green and yellow)and an extra secondary tap (orange)which is confusing
The schematic shows two secondary taps, 8Ω and 2.5Ω. No orange wire is shown. Check resistance between the orange wire and every other wire, one at a time. I'm thinking you will find no continuity to the other wires. But, you may find that the orange wire does have continuity to the transformer bolts or lams. If so, just connect the orange to chassis. Don't try to use the orange as a secondary tap.
And another green and yellow tap which seems to produce approx 120 VAC
That's probably a bogus reading. Check resistance between that green/yellow wire and the filament wires. That's likely a center tap for the heaters. The schematic shows the heater center tap connected to the cathodes of the output tubes. This is for heater elevation.
Manual with schematic is attached...
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You have a schematic! I was looking for one for a while the other night. Clearly you are a far superior searcher than me in every regard.
and duhh on the heater centertap apparently my brain is not working. I haven't played with amps for a while. I should've known taht from the colours. I will peruse the schematic. hmm it does have 324 at the power tubes. Though I measured 420-430 at the first cap(just checked it again) I guess with the load of the tubes that voltage will likely come down considerably.
I think it was that the bogen used EL84s that made me consider an El84 Princeton
Will this lower voltage make a sad and weak PR? The VAC coming off my PT is 320 320
The PR PT on Hammonds site is 325 325
On an up note the stereo double deluxe type amp based on 7189s I built a while back has played a bunch of gigs, more rehearsals and even a couple of recording sessions and is still sounding great and going strong.
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I have the chassis mostly reconfigured with a new face plate though still part of a hole to patch.
I was looking at various one tube reverbs
-There's tubenits favourite using a 12AX7
-I've also read a 12DW7 makes a good reverb tube using the 12au7 side as the driver, though I can't find a firm schematic for one. I did find this transformer-less circuit
-I also have a 6BM8 tube which most of these (see picture) use.
Any recommendations if I wanted to go the one tube route. I am used to a Fender sounding reverb however I only use full reverb in one song(as an effect) and this amp might not even see that application, other wise I mostly use it between 1 and 4 max.
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I think those 6BM8 transformer circuits will have a Fendery sound. The 12DW7 will probably sound like an old Ampeg, which has a nice sound but not quite like the Fender sound.
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I mostly use it between 1 and 4 max
IF you use a Fender reverb between 3 to 4, then I am not sure you'd be happy with a 12AX7 one tube reverb? The PR I owned had ALOT of reverb in it between 3-4.
I like Sluckey's suggestion to try a 6BM8 reverb. :thumbsup:
With respect, Tubenit
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Thanks for checking those out!
To be truthful I haven't used an amp with actual numbers around the knobs (or on them) for a couple years now . Likely more like 1-3 with 3 being a rarity. Probably like yourself I find the everyday useable range of fender reverb to be more around 2 max.
Someone on the Gretsch forum was telling me one could use a 12DW7 with the existing Fender circuit. 12Au7 triode as a driver and 12ax7 triode for recovery. They said it was less reverb off the bat but still more than enough when turned up.
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I realized that I could check this one tube 12DW7 reverb driver and recovery in a fender BF reverb circuit concept out myself.
The relevant pins on the tube socket are just tied together for the reverb driver tube.
So if I could disconnect them and put in a 12AU7 I'd get an idea of how it would work.
Looking at the socket in my BF vibrolux build, it I thought what PITA to desolder! then I remembered I have LOT'S of 12au7, like about 30 and those were just left overs from the tube gluttony of my misspent youth.
So I made a half 12AU7 using a pair of snips, carefully plugged it in and it seems to work quite well with plenty of nice sounding reverb.
I have a question. Is this potentially bad in the long run ?(flaming 12AU7. melted bits, zombie apocalypse , etc.etc.)
I wouldn't do this half 12AU7 thing permanently with an existing amp, it was JUST to try a 12DW7 one tube fender reverb circuit
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Excellent idea! :thumbsup:
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Excellent idea!
+1
That indeed was rather clever!
With respect, Tubenit
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Jeff, I think replacing the 12AX7 with a 12DW7 in your one tube reverb amps would be worthwhile.
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Snipping pins on a Mini begs for micro-cracks and air-leaks.
Maybe you are lucky. Maybe it is a very slow leak and won't quit for days.
Good experiment. Bad for road gigs.
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Jeff, I think replacing the 12AX7 with a 12DW7 in your one tube reverb amps would be worthwhile.
I will give that a try. I ordered one. Ironically, I had one in another amp but gave the amp to someone. I do have a reverse 12DW7 and have found that useful in one of my amps.
With respect, Tubenit
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Snipping pins on a Mini begs for micro-cracks and air-leaks.
Maybe you are lucky. Maybe it is a very slow leak and won't quit for days.
Good experiment. Bad for road gigs.
Hi , maybe you missed the end of that posting , where it says;
"I wouldn't do this half 12AU7 thing permanently with an existing amp, it was JUST to try a 12DW7 one tube fender reverb circuit"
The tube pin clipping was a one off, one time use to hear how the idea would sound. I wouldn't put a half tube in any of my amps permanently. I play shows pretty regularly and love reliable gear!
And for those interested in trying it the fellow that suggested this said he has built a few amps using this version of the fender circuit and it holds up well in the long term.
For kicks here's some video of my stereo deluxe that I built (with help here) in action live.
https://www.youtube.com/watch?v=0dKUMxYaXQ8 (https://www.youtube.com/watch?v=0dKUMxYaXQ8)
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Hi
I forgot to ask
IS the half tube tremelo going to be adequate for an amp that will end up more DR-like with the long-tailed Pair PI ?
Maybe just a little less deep?
Which also begs the question;
Since the power amp voltages and bias are actually pretty close between a PR and a DR
Is it primarily the transformer size and the PI that make the difference in actual power?
Thanks!
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Okay did further research and answered my own question. Thanks Me! :icon_biggrin:
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What about using a mosfet in the vibrato section? And if you used a ECC823 Reverse 12DW7, you could get by with 4 12A_7 type tubes.
Just a thought to consider. NOT saying you should build this. And I have not built this amp.
Maybe Sluckey will weigh in on this idea and add some corrections or edits if needed? I am thinking the 3.3M mixing resistor might need to be reduced to 470k or something like that?
With respect, Tubenit
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Toxo - loved the vid seeing and hearing Sarah along with the band's flair. The lady's got it and great style not easy to find. A keeper! ;)
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Toxo - loved the vid seeing and hearing Sarah along with the band's flair. The lady's got it and great style not easy to find. A keeper! ;)
Thanks very much! She's actually singer number 4 over a Looonnng period of time. They've all been good singers! Sarah's good too and has fun live.
Guys always like her cause she's such a looker!
For me. as the writer, the music is the thing, but having a good front person never hurts the live show that's for sure.
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Is that the Hollywood Vampires?!?!?! I think I see a young Johnny Depp on guitar! :icon_biggrin: Seriously, very nice chops and a great tune! I missed this in your build thread, what speakers are in that stereo rig? I see some silver dust covers...? Sounds great!
Jim
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bahaha If I hear that Johnny Depp thing one more time heads will roll!!! :icon_biggrin: He stole my facial hair...though I stole it from Errol Flynn.
In the stereo amp. Main amp (dry) - JBL D123 , Echo amp - Eminence little texas.
Amp has more than enough clean 'headroom' for this band and happily plenty for my louder rock band too.
I'm glad you liked the video
If you like the music there's another 10 videos up from the same show and more to come. Here:
There's a fair amount of musical variety.
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A layout queation
just establishing where things go in the chassis.
I made a long coneboard and thought maybe I could have a little one to the side to do the rectifier and maybe the bias circuit on (or the first filter cap)
Do the cones on teh little board near the side of the chassis look far enough away from the side to hook the transformer Ac up to? THere's an 1/8' between the edge of the cone and the edge of the chassis. Seems like it'd probably be okay.
-The other thing is does it matter really where I do my main power amp ground?
Generally I would have it at that end of the amp near the AC mains jack and the PT (tube side of chassis)
I have a little more room on the control side of the board so i generally run the B+ and the grounds down taht side of the board.
- Is it a big deal to have the main power amp ground on that side , near the end of the pilot light ?
Just considering some options and looking for a bit of input.
Thanks!
oh and some progress sockets, boards, faceplate, power supply stuff. Still have to put in the turret strips and another hole for the reverb transformer (which I'm waiting on).
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bump!
and I'll add another question regarding cathode bypass caps and resistors
On the standard fender layout the reverb recovery stage and the 'post reverb gain/mixer stage' share a cathode 820 ohm resistor and 22/25 bypass cap.
On my build the recovery stage will being the 12DW7 tube and the post reverb gain stage will be sharing the next 12AX7 with the tremelo circuit.
in the interest of sharing board space is it possible to jumper the cathodes of two different tube gain stages together and have them share a bypass cap and resistor? Too weird??
I can give them separate caps and resitors (1.5k and 22/25), it's just that board space is at a premium
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in the interest of sharing board space is it possible to jumper the cathodes of two different tube gain stages together and have them share a bypass cap and resistor? Too weird??
No reason you can't do that.
I can give them separate caps and resitors (1.5k and 22/25), it's just that board space is at a premium
I'd rather do this.
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Did the second option and thanks.
Hey, I have two output taps on my OT and I was wanting to access them both when needed.
In the past I have used a big switch to be able to have speaker options.
This time due partly to space I was installing two speaker jacks ( i could still install a switch)
i then realized both my jacks were the type that shorted out when unplugged.
it seems like this could be a problem for an unused tap. Is that right?
I could either use non shorting jacks or just go back to the switch idea, there's probably enough room.
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No problem using switching/shorting jacks. Just don't connect anything to the switch lug.
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duh!
Thanks
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Researching wire gauges on the internet as I'm wiring up the B+ 'rail'
I have some solid 20 AWG however it's insulation is rated at 300V
I've read that many people use that. It certainly is easy to work with.
there is however a lot of conflicting opinion(as usual)
I also have some automotive 18 gauge automotive primary wire, stranded and rated at 600 V
It's much fatter with fatter insulation.
As a result it takes up more space on terminals and doesn't run as nice
A little harder to work with
Is it necessary in a guitar amp to use the 600V rated wire?
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Wire ratings are more about expected max current to be flowing in it and used in a given distance. Also how and where it's to be used. If the application is expected to take a lot of large/heavy vibration then stranded is more appropriate due to it's ability to withstand more movement and is more flexible with less chance of complete breaking failure over solid core.
Being that we're only using very small couple foot runs max from the PT to any of the reservoir/filter caps is usually only a matter of inches. Our AC part of the circuits uses very little current so no need for very thick wire. This wire doesn't need a huge amount of insulation as our amps in general do not take a lot of abuse due to wear nor is in environments under ground, direct burial, etc. AC wiring is usually always stranded wiring and why you see it used in those applications (think heater wiring also). The DC parts of our circuits are usually solid core due to ease of working with it
In contrast the car/boat/trailer wiring is much lower voltage but at much higher currents. This wire needs much larger gauge to minimize the voltage drop due to long runs, heavy insulation and to be stranded to withstand the constant and harsh vibrations and movement from driving and traveling through rough terrain and environments.
Additional info to think about: By the time the voltages make it to our preamps going through a number of filtering dropping points, it has lowered considerably. Then it drops even more after the load resistors to the plates of a 12ax7 per se. This can now be less than 200v? Yet many still use 600v capacitors all the way through the circuit. Why? There's no need. They're paying more and taking up more space unnecessarily. Same when all 1/2w resistors are used throughout. Look in any modern Fender, Marshall, Vox, etc - circuit boards and you'll see very small 1/4w, 1/8w, and surface mount devices being used.
In summary: It is prudent to use only the power ratings that are necessary for the application. The standard "good practice" is to find out what the actual wattage is going to be running through any given component and then double this rating for safety and reliability. Think power resistor here: measure the voltage drop across the resistor in question > then square this number > divide the actual measured value in ohms > then multiply by 2.
Example: "D node" of preamp measures 250V to a 100k load resistor and the other side measures 95V. 250V - 95V = 155V drop. 155 * 155 = 24,025. 24,025 / 100,000 = 0.24W, then double this result (0.48W) for safety and you can use the nearest standard resistor which is a 1/2W 100k resistor for that application safely and reliably. It should not even get warm while in use.
Many builders will use all 2W sized resistors for their preamp load resistor circuits just because they either have them and it looks cool to do so or maybe they like to use all wire wound and that's what they have in stock and feel that lowest possible noise will be achieved, because as far as resistor noise is concerned wire wound is found to be the quietest. This is and can be an important consideration especially at the earliest point in the circuit where if anything like this exists? It will then be re-amplified down throughout the rest of the entire circuit more and more through each stage to the power tubes. Just more "food for thought" as my dad would say. I suggest that for any more theoretical questions like you are asking to be made into new threads for others to see and learn from. This information should have it's own topic and keep Princeton questions to the Princeton thread.
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Inside a chassis (not in-wall or exposed), the 300V stuff is good for far more than 300V. I'd use it anywhere except the OT plate leads.
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Thanks!
I have a grounding question.
Looking a Doug's grounding page.
I'm seeing the first two filter caps and the power tubes cathodes, PT center taps, OT, speaker jacks and the bias supply all grounded to a ground point bolted to the chassis near the PT
And Everything else is grounded to a point near the input jack.
Is that the right interpretation?
I must admit on most for my builds I've been including the PI in with the power amp grounds, as well as the reverb driver circuit and the trem circuit (which all get their juice from the same B+ node.
Would it be better to have the entire reverb circuit and the PI on the preamp ground point.
Is this bad practice?
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Take a look at some of Hoffman's layouts for his builds & how he does the grounding in those in the Library of Information:
https://el34world.com/schematics.htm#Hoffman_Blues_Junior_Conversion
https://el34world.com/Hoffman/files/Hoffman_AC30.pdf
https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf
With respect, Tubenit
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....... I've been including the PI in with the power amp grounds, ......
If the amp has a -FB loop from the OT secondary to the PI, then you can/should ground the OT with the PI ground point. (See link below, pages 273 and 274.)
as well as the reverb driver circuit and the trem circuit (which all get their juice from the same B+ node.
Generally, you would ground a circuit to/with the B+ filter caps ground that's supplying the B+ to that circuit.
Here's the link to Merlin's web site on grounding;
http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html)
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Cool and thanks!
Interesting looking at the Hoffmans schematics linked. It seems like Princeton and the AC30 include the PI with the power amp grouding buss while the Bues Junior has it part of the preamp grounding buss.
As the Princeton,being the only one of the three to have trem and reverb and it uses the Preamp ground for both these sections though they are supplied with B+ from b which is the filter section for the Power tube B+
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Jeff, I think replacing the 12AX7 with a 12DW7 in your one tube reverb amps would be worthwhile.
I will give that a try. I ordered one.
Have you tried this yet? Your thoughts?
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Are you asking Tubenit ?
I'm interested how it works in his circuit too
In essence I've tried it woth my nutty experiment, and it seems to work well, as has the fellow who suggested (albeit with a proper 12DW70 it to me, in a few builds.
I'm waiting on my reverb transformer and 12DW7 to arrive, probably this week some time.
In fact my build is almost done, but I need to install the reverb transformer before i complete the board hookup on the preamp end.
If the transformer arrives and the tube is looking to take a few days Ill probably just put a 12AU7 in just to try the whole thing.
I have two 12DW7 coming, one a JJ,-new and one from a fellow I know who is sending me a NOS Mullard as he has a raft of them.
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Have you tried this yet? Your thoughts?
After reading your post this morning, I gave it a try. I compared a Tung-Sol 12AX7 (which is what I had in there) to a JJ 12DW7.
Very significant difference in tone. I left everything dialed the same on amp and guitar & simply switched tubes and played the same "jazz" chording and then blues single note and double stop riffs.
I had the reverb pot set to "6" & then tried it around "7.5 to 8".
I don't hear much difference in the amount of reverb. About the same springy boingy sound with both. However, the 12AX7 adds some unpleasant "hash" harsh echo in the verb that the 12DW7 doesn't add. The 12AX7 makes the spring in the reverb tank sort of rattle on top of the higher notes. The more you dial up the reverb pot the more noticeable and even unpleasant in tone it becomes.
The harshness is maybe 80% eliminated with the 12DW7. The 12DW7 has the same springy boing sound but without the harsh rattle artifacts that the 12AX7 has. And the tone is clearer and cleaner sounding to my ears. It gives the overall chording and single/double notes a smoother clearer tone, IMO. The 12DW7 may not be verbing the lower notes as much as the 12AX7 seems to add a little "mush/fuzz" to the lower notes?
I tried it with just the clean channel and then the OD channel engaged. See schematic below. The 12AX7 really had noticeable harshness on the clean channel when the reverb pot was around 7.5.
Please note that I am using a .003 into the dwell instead of the typical 500p to .001 that I usually use. This could be a factor in the 12AX7 having the harshness? OR ……… maybe the 12AX7 triode is overdriving the send signal too much adding harshness?
I am leaving the 12DW7 in there. I only tried one guitar with this using a semi-hollow body Tele (no f-holes) with H-S-H pickups.
Thanks for the suggestion! I think it is a worthwhile improvement. I would not anticipate getting more reverb with the 12DW7 but getting a smoother and cleaner sounding reverb with the 12DW7.
However, in some ways I can get more reverb with the 12DW7 because I can turn up the reverb pot higher and not have the unpleasant harsh echo riding on the reverb/notes. So because I can turn the reverb up higher and still have it sound good, perhaps one could say the 12DW7 allows more reverb to be used even though it doesn't seem to produce more reverb?
With respect, Tubenit
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Thanks for the report Jeff.
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Cool!
Good to hear that works out in a positive way.
A pair of Mullard 7247s arrived today but the electronics store says they haven't even played the order for my reverb transformer yet...If I'd known that I would've spent a few extra $$ and ordered it from this place back east who would've shipped it in a few days!!
They say it will arrive next week if they put the order in today...I did order it 11 days ago.
i have a couple of other small single ended transformers 3-5 watts. One measures 250ohms - 1.6ohms the other is 185ohms - 1.2 ohms
i guess I couldn't get by with either of these . From what I've read fender reverb transformers are about 2k - 1 .2 ohms
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If you had ordered from Hoffman you would probably had it in only 2 days.
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Perhaps I misunderstood this site's intent. Should I not be here if I don't purchase things from Doug's store regularly?
He has good prices etc, but shipping costs to Canada on individual items sometimes make them cost prohibitive. the exchange rate is terrible right now too though his prices reflect that to some extent. I think shipping on that transformer would be about as much Can as the transformer itself.
All respect to Doug and this excellent site.
Also I tend to still think go-to-a--store-and-buy-it rather than 'order it online' though it is probably more practical to order it online.
I guess that's a no on my question regarding the transformers I have??
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There has never been any requirement to shop in Doug's store. I was simply commenting on his fast shipping. Seemed appropriate to me since your source was dragging their feet with your order. I didn't see any transformer question. I'm a straight talker so you don't ever have to read between the lines or assume I meant something different than what I said. You've been around long enough to know that. :icon_biggrin:
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True enough and I guess my question was worded in a rhetorical fashion
here it is again
i have a couple of other small single ended transformers 3-5 watts. One measures 250ohms - 1.6ohms the other is 185ohms - 1.2 ohmsi guess I couldn't get by with either of these . From what I've read fender reverb transformers are about 2k - 1 .2 ohms
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Based on that info I would not use either of those transformers.
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Well because I'm mad and impetuous I wired in the larger of the two (size and primary resistance) reverb transformers and after the usual mishaps( parts backwards, electrocution, 1meg instead of 100K resistor, ooops no coax cable connecting the reverb mixer with the PI) I got the amp going and it sounds good.
However I guess as expected , even with an SS rectifier my voltages are lowish in the power amp, 350VDC biased to 25 on the 6V6s
170 on the preamps and 180ish on the PI
Had to do some messing around with dropping resistors.
it might be a thing if I want more om pah pah to put in my backup PT that produces 375-0-375
Right now I'm maybe getting around 16watts which isn't bad.
Humourously I have reverb with the wrong impedance transformer and the 12DW7. Fine sounding reverb, not quite enough to be Dick Dale but more than enough for everyday use . Still will get the proper transformer.
I have an OT question
the VP20 schematic and instructions claims there is an 8 ohm tap and a 2.5 ohm tap (for 3 speakers)
Measuring the turns ratio on the OT (variac at 10VAC on the secondaries)
I get 30-1 on the yellow tap (300vac)
and 22.7 - 1 on the green tap (227 vac)
It seems like the yellow tap would be the one to use with 6V6s (or EL84s as the original had) with a 8 ohm speaker
While with 6V6s or El84s the green tap would be more suited for a 16 ohm speaker (or pair of 8s in series what have you)
For now I have a merette on the unknown(maybe ground) orange wire on the secondary side.
I did attach it to ground while a guitar was plugged in, right after I strummed it. and the map got noticeably quiter
strummed the guitar and unhooked it from ground and the amp got noticeably louder
Could it be a NFB tap??
Compared to the schematic 9and instructions it doesn't really make much sense
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Could it be a NFB tap??
Use your ohm meter to get a clue about the orange wire. Check for resistance between the orange wire and every secondary wire. Check for resistance between the orange wire and every primary wire. And also check for resistance between the orange wire and the shell of the OT. What are your readings?
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Just so the proper methodical approach and one that I probably should've taken before building...
Here's some readings in a hurry prior to rushing off. When I get back I'll desolder the black secondary from speaker jack ground (and this chassis) to properly check the shell.
So for now
Resistance between the orange wire and all the secondaries (green yellow and black) and chassis would seem to be about .4-.5 ohms
Between the orange wire and the all the primaries (brown black red) it reads as open
As stated already(though in a couple of posts just trying to get all the info on one spot)
As far as testing for turns (with 10VAC) when the orange wire was considered as a tap and tested with the black secondary (briefly) the transformer buzzed ominously. It did the same thing when the orange wire was considered as a ground and tested with the green or yellow secondary.
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The usual crazy unconventional construction.
Seems to be working well
Probably achieving about 16 watts
Reverb works great
I found another Ot that I had kicking around. I think it's an actual hammond organ reverb driver. It seems to have the typical hammond AO sort of code A0-27311-1
500 ohms on the primaries and 1 ohm on the secondaries, seems to be working
Only issue right now is that my tremelo is making a bit of buzzy noise in time with itself, very quite but something to track down.
Now to finish up the new baffle and mount it in the old regal cabinet with the original logo so it's kind of a sleeper amp.
The usual assortment of knobs, One is a late 60s Gretsch knob off my Tennessean actually.
i have to go pick up a few more small ones.
Still don't know what the orange secondary tap is but thought I should leave well enough alone
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Here it is (mounted in the ratty little regal cabinet I had), with a friend.
I have a JBL D123 in it which sounds great of course. originally I had a lighter 12" Rola alnico speaker from an organ and it sounded really nice except it didn't have much oomph and would crap out on the bottom very easily.
The JBL is HEAVY though.
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:thumbsup:
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Well done! Thanks for sharing your work. With respect, Tubenit
:thumbsup:
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My pleasure, and more to the point, thanks for the help!
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If anyone is interested in hearing the one 12DW7 tube version of the standard fender reverb circuit I made some short sound clips.
Using the princeton deluxe reverb I just built with a 12' Rola speaker in it which has a pretty top end. To be fair the reverb circuit is using a Hammond AO-63 reverb driver transformer 600 ohms - 1.2 ohms and a fender type long 2 spring tank.
Possibly the circuit could provide even more verb with an actual fender spec reverb driver transformer.
I find there's more than enough for my uses.
Amp is miked with an ubiquitous Shure 58 because it was handy.
Guitar used is 1953 Gretsch Electro 2
Amp is set at V - 1.5-2 T - 3.5 M - 1 B - 3 and the bright switch is on.
First few clips are just a few different things played with what I would consider an everyday amount of reverb set around 2
Last clip is reverb on 10 full on and appropriate music...
https://soundcloud.com/user-650789422/prd-sound-sample (https://soundcloud.com/user-650789422/prd-sound-sample)
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Hey, thanks! I enjoyed hearing that a lot. You've got a very fun and engaging playing style. Both your playing and the reverb sound great.
With respect, Tubenit
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I am leaving the 12DW7 in there. I only tried one guitar with this using a semi-hollow body Tele (no f-holes) with H-S-H pickups.
Thanks for the suggestion! I think it is a worthwhile improvement. I would not anticipate getting more reverb with the 12DW7 but getting a smoother and cleaner sounding reverb with the 12DW7...
Hi Jeff
I take it the 'AU' triode (in the 12DW7) is the one being used for the reverb driver stage? Assuming this is so, you may get even better results if you increase the cathode resistor to get a decent centre-bias on this triode. Assuming a fender RT with a 25k load resistance in Class A, this would like to see about 14v or so bias giving 6.5mA plate current, (312V* x 6.5mA, is 2W plate dissipation = ok) 14V/0.0065A = 2,153R, so 2k2 cathode resistor ought to do it. 14V bias will give you more headroom.
*I'm crudely assuming a plate idle voltage of 312 - going off the voltage notations on your schematic (and of course that was with a 12AX7 triode in there). Of course, the plate-to-cathode voltage is the pertinent metric to measure, and so in reality, you'll probably be seeing something other than 312V, but the above calculation is enough to give you a ballpark idea, and you can adjust the cathode resistance once you see where it lands. YMMV
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so 2k2 cathode resistor ought to do it. 14V bias will give you more headroom.
Yes, the 12AU7 side of the 12DW7 is the driver.
As always, thanks for the information! I appreciate all that you contribute and share on the forum. :thumbsup:
With respect, Tubenit
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Interestingly, the 'AU' triode in the 12DW7 is rated at 3.3W, so theoretically you could go even lower bias voltage
So assuming HT=312V, and eyeballing the last graph on the Sylvania datasheet, a 25k load line gives you 12.48mA at 312V, which at the 312V abscissa equates to roughly -12V (2/5ths of the way between the -10 and -15 grid curves), so that's 300V plate-to-cathode. 3.3W/300V = 11mA for maximum dissipation. 12V/0.011A = 1,090R, so that's basically 1k1 cathode resistor (to be on the safe side with your 25k RT idling at 312V). The 1k1 should be rated at 0.5W minimum.
Hotter than I first thought :laugh:
https://frank.pocnet.net/sheets/106/1/12DW7.pdf (https://frank.pocnet.net/sheets/106/1/12DW7.pdf)
https://frank.pocnet.net/sheets/127/1/12DW7.pdf (https://frank.pocnet.net/sheets/127/1/12DW7.pdf)
(PS - given the 3.3W rating, methinks a pair of 12DW7 would make a nice low power PP stage complete with LTP)
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Are you referring to tubenit's circuit or the fender circuit in my amp?
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tubenit's circuit. (Sorry for the thread hijack)
But if you're going to adopt a 1-tube reverb utilising a 12DW7 with the 'AU' triode driving a Fender RT for your build, then the same sort of analysis applies.
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I've been using it for a few weeks now, It sounds great, and no smoke!
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bahaha If I hear that Johnny Depp thing one more time heads will roll!!! :icon_biggrin: He stole my facial hair...though I stole it from Errol Flynn.
In the stereo amp. Main amp (dry) - JBL D123 , Echo amp - Eminence little texas.
Amp has more than enough clean 'headroom' for this band and happily plenty for my louder rock band too.
I'm glad you liked the video
If you like the music there's another 10 videos up from the same show and more to come. Here:
There's a fair amount of musical variety.
links broken.
--pete
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Well here's another one then
It must have something to do with this site as the youtube channel with all the videos is still there. THough looking at this one I'll have to talk to my drummer about the sync for the first shot...ooopsy something got nudged in the editing
Click on the bold band name in the description to be taken to the channel