Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ShaneCW on July 23, 2021, 10:31:27 am

Title: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 23, 2021, 10:31:27 am

Hello all, cant find a thread about this or the spec sheets in google searches for Fenders 047395 transformer.

A buddy gave me a HRD to look at and I quickly found the PCB was toasted. Started to plan a 5F6A conversion or maybe a RR763R Blackvibe Reverb or an AB763 when I found the transformer to have the numbers for export 240v (047395). I cant find a spec sheet for this anywhere. I see people selling similar or cross referenced transformers with data but nothing for this actual transformer. I reviewed the HRD schematic and found this Xformer to be configurable to 100V/230V/240V. I've also seen people identify it to be configurable to 115V, but that is not identified on the schematic. This was connected to the board similar to the 100V but with Black/yellow and black/red P5 & P6 connections swapped. My guess is that is the 115V configuration... (I dont like guessing though). Serial number decoder told me the amp was from 1991.

If anyone has any documentation for this transformer or suggestions on how to jumper the primary side so I can determine what build I can use this for, and how to use it, I would greatly appreciate it. My next step would be to lay out all the windings (safely of course) use a variac to supply power and measure the secondary to find the right turn ratio. Load rating for the filament circuit would be helpful to know so I can verify if I can add an extra preamp tube as well as the load rating for the high side of the secondary. (If I do a 5881 or 6V6 amp, I believe I'll be under the ratings but still need to verify that. The 5881 at lower voltage might pull too much..).

I am a generator tech by trade and am skilled in electrical safety. I have never spec'd a build bottom up or retro fit a tube amp but I have successfully repaired a dozen or so amps, some of which were way above my skill level and were accomplished with great effort and frustration  :laugh:  I'm guessing this PT is going to be another one of those...

My buddy is broke so a new PT is not an option. Plus the challenge of finding a practical use for this PT kind of drives me a little.

Thank you for your time.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: shaun on July 23, 2021, 01:30:19 pm

You can safely assume that Fender was using the appropriate primaries for 120VAC. That is, if the amp was bought in the US. So I'd simply use their primaries.

As for the heater draw, you can do the math on the tubes that are in the HRD. Add the heater current draws from appropriate tube data sheets, and you'll know what the PT can safely handle - I think you're looking at two 6V6s, three 12ax7s and a 12AT7. Something like that. You might find you can go slightly over that rating in a new design, but not too much, or you'll have a very hot transformer. Within 10%, you'd probably be okay. But there is no guarantee, of course, because it was purpose-designed.

You can also estimate the HT draw of the tubes by doing the same tube math. Might need a calculator for that one.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 23, 2021, 03:59:48 pm

Thank you for the reply.

The information I am looking for is the amperage load rating for the 47395 transformers secondaries. Once I know how much load the transformer can handle then I will be able to calculate filament and plate loads of the different amps I am considering cloning. Inside this amp is a sticker near the PT that indicates 100VAC. I removed the PT, configured the PT on the bench with jumpers, fired it up on a variac and took secondary voltage readings at 100VAC and 120VAC. So now I know my peak operating voltages. The only thing Im really willing to assume is that the PT was engineered to run a 6L6 pair at 100v, which would make it stouter. (Ohms law: lower voltage with the same resistance increases amps).

How much stouter is the question? I would feel much more comfortable seeing a factory spec sheet or hearing from anyone who has used this PT in a project. Google has been no help. I also emailed Fender and got no reply.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: octal on July 23, 2021, 04:17:33 pm

From the Hammond replacement transformers list:

117V @ 60 Hz[/t]308.5V @ 250 mA39V @ 250 mA6.6V @ 3.75A HOT ROD DELUXE*021-3202-000240V @ 50/60 Hz308.5V @ 250 mA39V @ 250 mA6.6V @ 3.75A As you can see, the secondary ratings are the same regardless of if it's a domestic or international transformer.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 24, 2021, 10:46:35 am

The aftermarket export isn't configurable. One of my concerns is, when the primary is configured to different voltages, that the load rating of the transformer will be different for each configuration; 100/230/240. But, for all intents and purposes, I suppose its safe enough to assume that the Fender export PT is rated at 250ma with 308.5v on the secondary when configured to 100v, 230v or 240v on the primary like the domestic and export aftermarket PT's are.

Assuming that, running 100v configuration at 120v (which is what line voltage is at my house) will increase my voltage on the secondary and give me a lower current 'rating' because increasing voltage will decrease the amps on circuits with the same load. That means I'm running 120% over and need to reduce the assumed amp rating accordingly, which works out to 208ma, which is too low a spec for the 6L6 set up that ran in this amp for 25 years... There in lies my concern that this PT doesn't have the same ma ratings as the after markets, to me, this indicates that it is rated higher.

I measured 338v on the secondary with the rated 100v on the primary while in the 100v configuration and 405v on the secondary with 120v on the primary in the 100v configuration. I'm measuring more than 10% variation from the secondary output voltage on my PT as I am seeing on the specs for aftermarket replacement PT's while supplying rated voltage. So that indicates yet another difference...

My complete readings for all secondaries were: @ 100vac: 338v, 7.1v, 41v; @120vac: 405v, 8.4v, 49v. Also notable: this PT was factory installed with a sticker on the chassis next to the PT indicating it was configured to 100v.


If there is anyone out there with the spec sheet for this PT it sure would help me answer my own questions.

Thank you for your patients and replies!

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 24, 2021, 10:54:57 am

Also, my P5&P6 connections are just a jumper and doesn't change anything swapping them. Looking at the schematic and following PCB tracings showed me how to connect this to a variac for bench testing in the 100v configuration. I have not found a configuration option for this PT in 115v.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: sluckey on July 24, 2021, 11:36:51 am

I would not configure that PT for 100V operation and then connect it to 120V wall voltage.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: sluckey on July 24, 2021, 12:03:42 pm

Take some voltage readings on that PT and we may be able to figure out if you can configure it for 120VAC operation.

First connect 6.3VAC to the two green wires. Then connect your black meter probe to the BLK wire. Using the red meter probe, measure and record the voltages on BLK/YEL, WHT, AND WHT/BLK wires.

Now connect your black meter probe to the BLK/GRN wire. Using the red meter probe, measure and record the voltages on BLK/RED and VIO wires.

Report your findings.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 24, 2021, 05:24:02 pm

Supplied 6.36vac to the green wires:

BLK wire to blk/yel:    102vac
BLK wire to white:       17vac
BLK wire to white/blk:  8.3vac
BLK/GRN to BLK/RED: 101vac
BLK/GRN to VIO:        101vac

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 24, 2021, 05:36:41 pm

HRD schematic identifying the 047395 PT does not indicate 115v configurable. Not sure if that matters.
I assume it was intended for Japan but ended up state side. I think my plan B will likely be to get one of those $40 120v to 100v transformers off amazon to keep with the amp. 75% cheaper than a new transformer... Plus hipster points for being Japanese import design  :l2:

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 24, 2021, 05:45:05 pm

...My other option might be to get a 7v transformer for the filaments and find an amp that needs 400v supply...

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: sluckey on July 24, 2021, 07:14:24 pm

Try this hookup. If you get about 6.3 between the green wires without smoke, then the other voltages should also be close to what you want.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 25, 2021, 08:22:20 am

You lost me at 'if you see smoke'  :laugh: Would prefer not to risk melting the insulation off of the wires or shorting this out. Which seems possible considering this PT has not been proven to have a factory 117 configuration. The load rating of this PT would also be unknown and I would be concerned having so many open leads on the primary that when I put this in circuit that it would overload and short.

Do you have an opinion about using a bucking transformer in front of the HRD's PT?

I asked Luigi at Luigi Retro Customs about this transformer for use with his HRD2ODS replacement board and he suggested a bucking transformer and went into detail with his explanation. Ultimately he suggested to get a 50W 120V to 20V toroidal PT from AnTek. Which measures 3.6” diameter x 1.6” tall, weighs 2 lbs and costs $19. Then provided drawing and a simulator snap shot.

Unless someone sees something he may have missed or overlooked, it will likely be the route I go for use with this PT. To me, this looks like the safest and cheapest option for use with this transformer in a 2-6L6, 3-12AX7 conversion. With my skill level and the help he has given me, as well as his reputation for being helpful, it looks like his board will be the way I go with this project. I'll just save a point to point clone for the next one...

Here is a link to his store:
https://luigiretro.com/

...and here are the drawings and email conversation I had with him:

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 25, 2021, 08:35:09 am

For posterity, here are other relevant attachments; including HRD schematic with 100/230/240v export connections, Classictones 40-18015 which is the HRD replacement domestic PT.
Thank you.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: sluckey on July 25, 2021, 09:08:46 am

Quote from: ShaneCW on July 25, 2021, 08:22:20 am

You lost me at 'if you see smoke'  :laugh: Would prefer not to risk melting the insulation off of the wires or shorting this out.

It was a joke!!! There's nothing dangerous about the test I proposed.

Why are you chasing other transformers? I thought your buddy was broke? But if buying another transformer is an option, then quit chasing a HRD replacement and just get a more appropriate PT for the conversion you have in mind.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: PRR on July 25, 2021, 03:06:43 pm

Quote from: ShaneCW on July 25, 2021, 08:22:20 am

... I would be concerned having so many open leads on the primary that when I put this in circuit that it would overload and short. ...

Maybe I don't do enough power work. How are "open leads" a danger?

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 25, 2021, 04:22:56 pm

I couldn't tell if it was a joke, sorry. Adding a $25 transformer is much more feasible than a new $150 PT, but being able to use this on its own would be most desirable as long as its not going to burn me in the end. I just don't have the confidence with this thing not being labeled for 120...

I could be thinking of this wrong but, not utilizing jumpers could we be leaving a set of equal turns out that may be running in parallel with another set of windings to help increase the load capabilities in the lower voltage configuration? Where as running in series through them all would be for the higher voltage configuration. Higher voltage draws less amps to achieve the same power as lower voltages; Higher voltages requiring less copper/thinner wires on the turns. I believe as long as all the wires are the same length and the same number of turns, magnetism will keep the sine waves in phase from the split point through their identical turn ratios. Google tells me this is possible but google says a lot of things... Again, I could be wrong on this. I don't know enough about transformers to say for sure about how reconfigurations actually layout inside. If that is true, then configuring to 120 by using only 1 set of turns could lower the power capability of the PT. My fear is it would bench test fine but as soon as I put a load on it, that it would let the smoke out.
That's my concern...
Feel free to slap me if that is hog wash.

Thank you all again, I appreciate it.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: sluckey on July 25, 2021, 05:28:17 pm

It's hogwash! Slap yourself.    :BangHead:

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: thetragichero on July 25, 2021, 06:17:13 pm

shipping is a lot because it's from australia but what about this?: https://reverb.com/item/11036284-fender-hot-rod-deluxe-120-volt-transformer
this one is slightly less and brand new: https://reverb.com/item/27732925-transformer-fender-power-hot-rod-deluxe
sell the one you have in your hands and you'll probably be not much more than the imperfect $25 bucking transformer solution
or find something that suits what you're trying to build

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 25, 2021, 06:32:05 pm

LOL right on. Thank you again for the help.

I configured the PT with Hot to Violet and N to Black with black/green & white jumped, I supplied 120v and measured 6v on greens, 35v on browns and 290v on reds. Looks like its running 10v under spec on the Reds, 4v under on the browns and .3 under on the filaments.

When the filament circuit loads that may drop a little more, and then sometimes the grid may send a little less depending where you are, do you think its ok putting out 6v for the heaters on the bench like this? Or do you think running the tubes colder may cause issues? I haven't seen a lot but Im more use to seeing a little higher voltage on the heater circuit than lower.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 25, 2021, 06:34:29 pm

*by spec I am comparing to Classictones replacement 40-18015

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: ShaneCW on July 25, 2021, 06:39:32 pm

Quote from: thetragichero on July 25, 2021, 06:17:13 pm

shipping is a lot because it's from australia but what about this?: https://reverb.com/item/11036284-fender-hot-rod-deluxe-120-volt-transformer
this one is slightly less and brand new: https://reverb.com/item/27732925-transformer-fender-power-hot-rod-deluxe
sell the one you have in your hands and you'll probably be not much more than the imperfect $25 bucking transformer solution
or find something that suits what you're trying to build

I think I would go the bucking transformer route before buying one of these, simply based on the savings. Thank you for showing me these.

Title: Re: Roached Hot Rod Deluxe w/ export transformer 047395 for conversion to...
Post by: sluckey on July 25, 2021, 08:21:44 pm

Quote from: ShaneCW on July 25, 2021, 06:32:05 pm

I configured the PT with Hot to Violet and N to Black with black/green & white jumped, I supplied 120v and measured 6v on greens, 35v on browns and 290v on reds. Looks like its running 10v under spec on the Reds, 4v under on the browns and .3 under on the filaments.

Jumper BLK/GRN to WHT/BLK will probably suit you better.