Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on February 02, 2019, 05:35:02 pm
-
Hi guys, A bit of an unknown Fender student level amp that looks to have some hidden talent.
The interstage transformer is also intriguing.
This is where it is a bit grey on the info.
I am unable to find any specs on the transformer and Evatco (local supplier)have a limited range.
The Stancor equivalent is 1:3 ratio, so I'm hoping that will work.
There are some modified versions of the original circuit to convert it from bass to guitar.
(https://i.imgur.com/ti9jHt5.jpg)
(https://i.imgur.com/hvE849A.jpg)
(https://i.imgur.com/5k08ELr.jpg)
I'm not sure there is a version of the amp as per the bottom schematic, most of the amps only have two knobs.
I would think it would loose a bit of signal and a bit natural growl that is in the clip.
Any thoughts would be great.
-
Pretty sure any additional circuits like that reverb would be on a modification. Theres two different versions of these amps the only difference really being the output tubes. They are pretty sweet cheap fender amps and a lot of folks use them unmodified, or with an upgraded speaker. I did look at possible mods to these amps but I didn't trust its available current to add another tube but I may be wrong. Are there examples on the net of someone actually playing a musicmaster bass with reverb?
-
Hey mate, The reverb is a mod I like to add to my builds, so it is not part of the original circuit.
I think 6AQ5 power valves were used then change to 6V6s.
-
I like inter-stage PI trannies, did 2 amps with them, you get a nice secret sauce using them :icon_biggrin:
as for your reverb, you are the reverb add-in expert :icon_biggrin:
-
So you don't need 16 cascaded gain stages for good distortion?!?! Gasp! Who woulda thought!!!!!! :icon_biggrin: That's cool, and what, about 10 parts? Timbo, you need to build this!!!
Jim
-
Thanks guys.
Shooter, I'm not too schooled when it comes to the interstage transformer.
Love REVERB :icon_biggrin:
-
schooled
Current kills 'em
now you're schooled :icon_biggrin:
they usually have a very low spec, mine are 10mA. so maybe measure vdc at V1B's cathode (470r) to calc I.
other than that, it's about 15 solder connections and a wad of parts less than an LTPI
-
Timbo.. Classic Tone has a replacement and I'd bet they would give you specs if you asked.. also don't know if they'd work but there were interstage trannys in Hammond organs..
http://www.classictone.net/40-18093.html (http://www.classictone.net/40-18093.html) they say it is 15 watt
I also found this that has some specs https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma
-
OK, here is where I'm at...
There has been some mixed reviews on this one, so a bit of head scratching has been needed to have a backup plan if the interstage transformer is a no goer.
(https://i.imgur.com/x5VWULJ.jpg)
(https://i.imgur.com/mFAvtmZ.jpg)
I have laid out the circuit board to accommodate some extra components that will make it more like a 5E3
(https://i.imgur.com/7fACp4q.jpg)
(https://i.imgur.com/Pd8YrQc.jpg)
All components can be added even with the board in place.
A 6AV6 will be added for the phase inverter.
-
.
-
What power transformer is being used? I see that you have modded yours using a different one as you have switched to one that supports a 5V rectifier tube.
I built one last year, but mine was only slight modded from the original, So I used the solid state rectifier and the replacement transformer set for the Musicmaster from Classictone (PT = 40-18100, OT = 40-18101, Driver = 40-18093). I too have left plenty of room on the board for further modification but, in my case, because I started with a little Acoustic G-20 as the donor amp, my chassis would be the more limiting part for me to expand should I later decide to redo with added tubes (as well as the space around the speaker when mounted in the cabinet). But to do that even, I would have to change the power transformer or add a small filament transformer somewhere to augment as the 40-18100 PT only delivers 1.1 amp (and it runs pretty hot even with the one 12AX7 and two 6AQ5's I used).
Here was mine as built: http://gwtekman.x10host.com/MM-12/ (http://gwtekman.x10host.com/MM-12/)
-
Awesome stuff mate! How does it sound.
The PT is similar to a Hammond 290AEX.
Got some time to continue....
(https://i.imgur.com/Kv2bwLh.jpg)
(https://i.imgur.com/xVo1jpj.jpg)
10 guesses to the potential problem in this photo
(https://i.imgur.com/xves6WJ.jpg)
-
Back on the job guys...
(https://i.imgur.com/L1VuvNg.jpg)
(https://i.imgur.com/RGA4H84.jpg)
(https://i.imgur.com/8yW1ABs.jpg)
(https://i.imgur.com/FTbe3cc.jpg)
A few untidy wires, these have been left long to get the phasing right if needed.
(https://i.imgur.com/oJmeazw.jpg)
(https://i.imgur.com/3n76x7c.jpg)
-
Those component and wire colors really pop inside that black chassis. :thumbsup:
-
As usual, great work Timbo. This amp was sort of looked over as Vibro Champs started fetching over $400. These would sell here in Atlanta GA at the same time for $100 to $150, so I ended up with 6. 4 had 6v6 and 2 had (early brain fart) the little version. I could look it up, but you get it.
The cabinets on the later ones were much more cheaply made. Little sleepers. BTW, they are bringing near $400 now. They are actually a great small gigging amp if your drummer is a wimp!
-
Thanks guys.
Amp fired up well, voltages were a bit high so a dropping resistor adjustment has got it at a better place.
(https://i.imgur.com/tzS26RM.jpg)
(https://i.imgur.com/GU20VgR.jpg)
Sounds pretty good....
Just a low level hum that I have tracked down to the power amp.
-
Guys, A bit more progress....
(http://i.imgur.com/q4QY5JJ.jpg) (https://imgur.com/q4QY5JJ)
(http://i.imgur.com/koitCFt.jpg) (https://imgur.com/koitCFt)
(http://i.imgur.com/NySlDmb.jpg) (https://imgur.com/NySlDmb)
-
Lovely! That will be sick when complete. Hows the weight balance with that chassis mount?
-
Hi guys
This one has collected a mountain of dust.
So there was a hum that was annoying me, it was a ground loop with the reverb tank.
The two RCA's are grounded on the tank, so I had both plug shields grounded on the amp chassis.
Disconnecting one shield from the chassis did the trick.
There is still some room for tweaking....
-
Any update, sound clips?
Thinking about building a MM based amp myself….
-
Hey mate, This one has been shelved for awhile, still deciding whether I like it or not