Mojotone DR Power Transformer and keeping 6V6 power tubes happy

[joulupukki]

I’ve got a Mojotone Deluxe Reverb power transformer ready to go for another AB763 single-channel build. But before I get too far, I’m realizing that it’s a 354-0-354 @ 150mA PT (https://www.mojotone.com/Deluxe-Reverb-Power-Transformer-125P23B) which seems like it’s gonna be putting out a lot more juice than what’s normally in a DR with a GZ34 rectifier tube. Should I be using a 5U4 rectifier instead?

Also, it seems like the choke for a DR is usually 4H @ 50mA but the one being sold by Mojotone (Mojo778: https://www.mojotone.com/Filter-Choke-for-6V6-Amps-125C3A) is 4.8H @ 75mA.

I was originally thinking to use a Nichicon 47uF 500V for the reservoir cap but if I need to switch to a 5U4 rectifier, it looks like I’d need to stay under 40uF for that first filter cap.

Anyhow, I’m just wondering if it makes sense to make these adjustments or just do what Mojo does in their build of the DR?

 

[Scottcurry13]

That is going to give you a really high B+ on the plates. With a GZ34 you may be well over 460vdc depending on you wall voltage.

 

[joulupukki]

That is going to give you a really high B+ on the plates. With a GZ34 you may be well over 460vdc depending on you wall voltage.

Exactly. That’s why I’m asking the question on how to approach/mitigate this. I tried searching around to see reports of people burning up output tubes prematurely with their builds but didn’t really discover anything.

 

[chas.wahl]

I'd sell that sucker to one of the power-obsessed guitar amp enthusiasts (there are a ton of them), even at a considerable loss, and invest in more appropriate, trouble-free iron. It's not worth the time spent trying to turn a sow's ear into a silk purse.

Or, build an amp that can use 460 VDC appropriately.

 

[King Fan]

You're right. Frankly I'm getting tired of defending Mojotone in general but saying "watch out for their PTs."

Back of the envelope, I can get B+ as high as 470V with ~350 HT voltage and a GZ34. I can get that down to ~440V if I spec a 5u4GB. But am I right that the DR schematic shows 420?

I don't know a thing about DRs, but I'd be worried not only about tube life but about tone differences.

B+ mitigation is covered I think by Rob on his 5E3 mods page: Zeners, variac, bucking transformer, move to third-world country with tired squirrels driving power grid...

I also know nothing about chokes, but from my *limited* experience I think that 4.8H will work for 4H spec...

You say "or do what Mojotone does." Do they have some B+ reduction in their build?

 

[2L man]

If you install Choke after rectifier tube and before first filter capacitor this drop HV DC and also regulate and filter HV better. There are voltage/current graphs on rectifier tube datasheets for this kind LC filtering.

 

[King Fan]

If you install Choke after rectifier tube and before first filter capacitor this drop HV DC and also regulate and filter HV better. There are voltage/current graphs on rectifier tube datasheets for this kind LC filtering.

Good idea. I never researched that -- does it require a bigger choke?

 

[schmee]

You're right. Frankly I'm getting tired of defending Mojotone in general but saying "watch out for their PTs."

Back of the envelope, I can get B+ as high as 470V with ~350 HT voltage and a GZ34. I can get that down to ~440V if I spec a 5u4GB. But am I right that the DR schematic shows 420?

I don't know a thing about DRs, but I'd be worried not only about tube life but about tone differences.

B+ mitigation is covered I think by Rob on his 5E3 mods page: Zeners, variac, bucking transformer, move to third-world country with tired squirrels driving power grid...

I also know nothing about chokes, but from my *limited* experience I think that 4.8H will work for 4H spec...

You say "or do what Mojotone does." Do they have some B+ reduction in their build?

My stock DR's have all been at 450-460V on the power tubes. It can be a problem, but JJ's work fine with that.
Let's see...... in the last 20 years these were a '65, '65, '67BF and a '68SF. I often use a 5V4 rectifier instead of the 5U4GB. But neither one seems to give a huge drop in voltage anyway.
The tone on these amps has always been killer so I wouldn't worry about that. But before I changed to JJ's, new EH 6V6's would last about 3 months with maybe 4-6 gigs a month. So let's say... 50 hours. I went through 2 sets of them before I went to JJ's.
A vintage RCA NOS set blew first gig and took out an OT in the process.
Personally I like the low value filter caps, although I usually use 20uf in lieu of stock 16uf.

 

[King Fan]

My stock DR's have all been at 450-460V on the power tubes. It can be a problem, but JJ's work fine with that.
Let's see...... in the last 20 years these were a '65, '65, '67BF and a '68SF. I often use a 5V4 rectifier instead of the 5U4GB. But neither one seems to give a huge drop in voltage anyway.
The tone on these amps has always been killer so I wouldn't worry about that. But before I changed to JJ's, new EH 6V6's would last about 3 months with maybe 4-6 gigs a month. So let's say... 50 hours. I went through 2 sets of them before I went to JJ's.
A vintage RCA NOS set blew first gig and took out an OT in the process.
Personally I like the low value filter caps, although I usually use 20uf in lieu of stock 16uf.

Real-world voltages? No fair. It's highly possible that B+ tonal effects in the big-BF range don't work the way they do in my smaller and older amps, or are more desirable.

I'm one of those goofballs who doesn't like JJ 6V6s very much, but I'm also the weirdo who tries to hit Fender '60s B+ specs. I'm totally serious it may be goofy -- some vintage amp owners say Fender never hit 'em anyway.

And for sure, @schmee, I'd like to hear more about filter cap size and sonic effects in these amps. There must be about 1,000 DR owners and experts on here.

 

[joulupukki]

Back of the envelope, I can get B+ as high as 470V with ~350 HT voltage and a GZ34. I can get that down to ~440V if I spec a 5u4GB. But am I right that the DR schematic shows 420?

Yeah, it shows 420V as B+ on the schematic.

You say "or do what Mojotone does." Do they have some B+ reduction in their build?

I think their power filtering is pretty stock: https://www.mojotone.com/Amp_Kits/Blackface/Deluxe_Reverb_SCH.pdf

They (MT) were quick in answering an email I sent them about this and … it sounds like, in practice, it’s not going to be an issue. They say their stock build runs about 430V on the plates and with my single-channel it’ll likely be around 435V.

Given that @schmee reports good findings using JJ output tubes (which is what I was planning to use in the first place), I think I’ll probably be good.

About their choke, they said after tearing down the original choke they said there was room for improvement and so theirs is able to handle a heavier load.

 

[King Fan]

Yeah, it shows 420V as B+ on the schematic.

I think their power filtering is pretty stock: https://www.mojotone.com/Amp_Kits/Blackface/Deluxe_Reverb_SCH.pdf

They (MT) were quick in answering an email I sent them about this and … it sounds like, in practice, it’s not going to be an issue. They say their stock build runs about 430V on the plates and with my single-channel it’ll likely be around 435V.

Given that @schmee reports good findings using JJ output tubes (which is what I was planning to use in the first place), I think I’ll probably be good.

About their choke, they said after tearing down the original choke they said there was room for improvement and so theirs is able to handle a heavier load.

Good research. Good idea to contact Mojotone.

Regarding the whole power / filter deal, as I say, I'm not DR-aware. I see where Rob says, "The Deluxe Reverb is known for its under rated power supply which gives the amp a lot of feel and touch response but leads to a loose low end and even "farting out" at very high volume settings. The other AB763 amps like the Twin Reverb have higher output power supplies that do not suffer from the Deluxe Reverb's loose low end. You can firm up the Deluxe Reverb with a larger first filter capacitor or even an upgraded power transformer but you will lose some of its touch sensitivity and playability."

 

[schmee]

Real-world voltages? No fair. It's highly possible that B+ tonal effects in the big-BF range don't work the way they do in my smaller and older amps, or are more desirable.

I'm one of those goofballs who doesn't like JJ 6V6s very much, but I'm also the weirdo who tries to hit Fender '60s B+ specs. I'm totally serious it may be goofy -- some vintage amp owners say Fender never hit 'em anyway.

And for sure, @schmee, I'd like to hear more about filter cap size and sonic effects in these amps. There must be about 1,000 DR owners and experts on here.

I kinda like the JJ's myself. But I will say that, while they dont sound like 6L6's as some say, they have maybe 10% of that less smooth tone which 6L6's have. For those of us gigging and pushing a DR to it's limits, it's actually a good thing. Maybe if you are playing quieter solo or duo etc, the smoother 6V6's would be better. But I often wonder if a blind test a player would actually hear the difference.

I modded my 65 DR to run 6L6's a few years back. I just wanted to see after many others doing it. There are only a couple things you have to do to make it a Vibrolux really. They are almost identical size even. The Vlux uses the same low value filter caps as the DR. Of course I put a V'lux multi tap OT in it which is the one big thing that needed doing.

Frankly I was amazed at how different the amp sounded. I hated it actually. It was sterile, loud and lacked the character of 6V6's, even JJ's. Sounding much like my Fender Pro does though, which I only use for louder outdoor gigs.
This confirmed in my head how little JJ's sound like 6L6's.
After a gig or two I changed it back to be a DR again.

 

[joulupukki]

I've got a pair of JAN NOS 6V6 tubes (that came with the DRRI I bought used). I've tried them in this amp and don't really notice any improvement / difference in tone so since I know JJs can take a beating and are comparatively cheaper (and are matched for now) I'll run the JJs. One thing that I don't like about the set of JJs that I have in my B-Verb (v1) combo amp is that one of them is fairly microphonic. I made the mistake of using the bear claw retainers and wasn't able to return/exchange them. Dr. Z. has a YT video explaining how he uses some silicone damper rings and a spring tube retainer to alleviate that problem (link at the bottom of this post, 16 min 15 seconds into the video), but I haven't yet implemented it. If I have the volume cranked a bit and hit just the right note, I get some odd buzzing. If I hold onto the offending OT it won't do that.

So, I'm hoping that the new set I order for the B-Verb amp head (v2) might not be as microphonic and I'll swap them. But will probably order the rings and spring retainer clips and do that mod in the combo amp.

 

[joulupukki]

I kinda like the JJ's myself. But I will say that, while they dont sound like 6L6's as some say, they have maybe 10% of that less smooth tone which 6L6's have. For those of us gigging and pushing a DR to it's limits, it's actually a good thing. Maybe if you are playing quieter solo or duo etc, the smoother 6V6's would be better. But I often wonder if a blind test a player would actually hear the difference.

I modded my 65 DR to run 6L6's a few years back. I just wanted to see after many others doing it. There are only a couple things you have to do to make it a Vibrolux really. They are almost identical size even. The Vlux uses the same low value filter caps as the DR. Of course I put a V'lux multi tap OT in it which is the one big thing that needed doing.

Frankly I was amazed at how different the amp sounded. I hated it actually. It was sterile, loud and lacked the character of 6V6's, even JJ's. Sounding much like my Fender Pro does though, which I only use for louder outdoor gigs.
This confirmed in my head how little JJ's sound like 6L6's.
After a gig or two I changed it back to be a DR again.

I'm glad you brought this up. I'd thought about trying this out (running 6L6s) but hearing your conclusion of the behavior, I'll skip that. Thanks for sharing!

 

[schmee]

I've got a pair of JAN NOS 6V6 tubes (that came with the DRRI I bought used). I've tried them in this amp and don't really notice any improvement / difference in tone so since I know JJs can take a beating and are comparatively cheaper (and are matched for now) I'll run the JJs. One thing that I don't like about the set of JJs that I have in my B-Verb (v1) combo amp is that one of them is fairly microphonic. I made the mistake of using the bear claw retainers and wasn't able to return/exchange them. Dr. Z. has a YT video explaining how he uses some silicone damper rings and a spring tube retainer to alleviate that problem (link at the bottom of this post, 16 min 15 seconds into the video), but I haven't yet implemented it. If I have the volume cranked a bit and hit just the right note, I get some odd buzzing. If I hold onto the offending OT it won't do that.

So, I'm hoping that the new set I order for the B-Verb amp head (v2) might not be as microphonic and I'll swap them. But will probably order the rings and spring retainer clips and do that mod in the combo amp.

I have not had a microphonic JJ 6V6 yet fortunately out of probably a dozen. But have heard there are some out there occasionally.
I've also had situations where something else in the amp makes a tube microphonic, although most often it's a tube, especially if it's a pre tube.

 

[Snfoilhat]

I don’t see the point of the deluxe reverb as a 6V6 amp. Setting aside the historic importance or the business case for having an amp in the 1960s showroom that looked like it fit between the princeton reverb and the vibrolux reverb, you’ve got enough information at your fingertips already to decide that the DR isn’t very well designed. So it raises the question what is 22 watts going to do for you compared to something 12-18 that can get hundreds of hours out of 6V6s, or what sized room is 22 going to shine in that 30-40 in a lower voltage 6L6 amp won’t? Not a rhetorical question to argue you out of it — what are you trying to accomplish in your design?

For the player that knows the DRs flaws are exactly what they want, that makes a lot of sense. But i’d guess most of them don’t worry much about the electrical details since they’re already worked out.

In the 80s Fender redesigned the push-pull 6V6 amps with long tailed pair phase inverter for both the PRII and DRII. They both had solid state rectification and nominal B+ 400V so you can see the choice to go with a differently wound transformer.

Last bit to agree with schmee above: lead dress problems can show up as microphony in a tube without the tube itself having a problem.

Looking forward to seeing the amp come together!

 

[Scottcurry13]

I built a 5F6-A Bassman with 6V6s using an Allen PT that was rated similar to yours @joulupukki. At the time I wanted to be able to switch between 6V6 and 5881 or 6L6. I always had trouble with it and it never sounded right with 460vdc on the plates with either NOS or JJ. I sold that PT and got a Mercury Magnetics with lower B+ and got the plate voltage down to 420. It sounded better, but not quite there. I ended up getting a Brown Box because the MM PT had my 6.3v supply at 6.8v with my normal wall voltage of 124vac. Now I’m running that amp at 116 wall voltage (6.3vac on the filaments) and it sounds really smooth and sweet. The plates are at around 400vdc.

 

[joulupukki]

Interesting @Scottcurry13. Hopefully this amp doesn't suffer the same fate. If I could figure out away to give it enough room and figure out which of the Antek toroids to use, that'd be my preference since I had some really good luck with using one on a different amp in the past. Lighter weight, plenty of power, and no hum at all from the PT.

The thing that kicked me off in this PT journey was when I built v1 of my B-Verb amp. I used a Musical Power Supplies PT (https://www.musicalpowersupplies.com/app/download/7115014784/PT330M_REVD_DWG.pdf) and it seems physically noisier than what I think it ought to be. I've tightened the screws on its iron plates, mounted it with some grommets for some shock absorption, etc. Each of those remedies helped a tiny bit, but still I think it ought to be quieter.

Then, because I'm a "noob", I learned that the DRRI uses the same PT as what a Vibrolux uses. For Hammond, it's the 290CX (650Vct @ 207mA). That got me wondering whether a more beefy PT would get me closer to the output power a DRRI can do (in comparison to my B-Verb v1 – which it's not quite as powerful as a DRRI I temporarily had).

In any case, once I build up a new chassis for v2 of this same amp (I just need to figure out a clean way to cut my aluminum sheet), I'll go use a box break and it'll finally be going. I've already got the amp head cab fully built (width is just under 20" so it's the same width as a Princeton Reverb). I just wanted to know what kind of gotchas I might hit before just diving in. My goal is really to have an excellent-sounding AB763 that's around 20 watts that's physically lighter than a DR to tote around but at least as equally as powerful and as good as, or better sounding.

 

[Lowerleftcoast]

Hey Shock Brothers and Sisters, it is time for a bench test!

Throw that PT on the bench and grab some alligator clips. @2L man likes to tape the leads to the bench for safety sake. If you have a Variac bring it along.
First, measure the PT as you normally would. Primary - Black and White to wall or Variac. Measure the secondaries (HT you don't have to measure red to red because that is a lot of volts for some meters to read. Just measure red to red/yellow CT.) Measure the heaters as well.

Second, connect primary 100V tap and 230V tap to wall or Variac. Measure secondaries.

Third, connect primary 100V tap and 240V end to wall or Variac, Measure secondaries.

Measure wall voltage just before each test.

Share results with the Shock Brothers and Sisters so we can guesstimate the final voltage under load.

Hopefully, one of these alternative taps on the primary will result in some love and still keep the heaters in the ballpark.

 

[archetype]

Real-world voltages? No fair. It's highly possible that B+ tonal effects in the big-BF range don't work the way they do in my smaller and older amps, or are more desirable.

I'm one of those goofballs who doesn't like JJ 6V6s very much, but I'm also the weirdo who tries to hit Fender '60s B+ specs. I'm totally serious it may be goofy -- some vintage amp owners say Fender never hit 'em anyway.

And for sure, @schmee, I'd like to hear more about filter cap size and sonic effects in these amps. There must be about 1,000 DR owners and experts on here.

Trying for the schematic spec values isn't goofy. No matter what they actually shipped, the spec values are the baseline you use to understand what else varies when you deviate from the spec.