Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: neddyboy on June 15, 2020, 03:08:38 pm
-
Hello all! I have some left over parts from a '60s tube reverb, including the transformer & power supply that ran a 12AX7 and a 12AY7, 2 sockets, etc. I'd like to build a hi-voltage dual tube preamp for guitar. The old reverb circuit is diode rectified, and that got me thinking about a Marshall-y sounding pre. I'd love to get more of a plexi overdrive sound on my 18 watt clone. I looked at schematics for the Marshall 2203 preamp, but they're part of the full amp schematic, and I can't make the leap from schematic to turret board.
Can anyone recommend a design, hopefully something intended as a standalone preamp, and ideally one with good documentation. The parts layout is pretty key for me and my just-past-beginner skillset.
-
Look here...
https://el34world.com/schematics.htm#Tube_pre_amp_in_a_pedal_box_
-
Wow, that's appealingly simple Sluckey! And no need for a turret/tag board. Cool. Thanks for the suggestion.
-
A couple questions! Since this circuit is a straight up preamp, I'd like to use it as a mic pre as well as line level. Any reason I couldn't add a mic step-up transformer to the input? Would it be as simple as adding the transformer and an XLR input ahead of the input? Second, this circuit has a wall-wart power supply system -- can I just ignore the PS & filament stuff and use the traditional high voltage transformer from my trashed reverb?
-
Most things are possible. What reverb are you repurposing and do you have a schematic for it?
-
can I just ignore the PS & filament stuff and use the traditional high voltage transformer from my trashed reverb?
The answer is probably/likely yes but it would greatly help to actually see the schematic? Can you provide a link, please?
With respect, Tubenit
-
The schematic is in the link I posted.
-
Take a look at this guy's work. maybe get some direction----http://www.frontiernet.net/~jff/af2b/Dual_Channel_Blackface-style_Preamp_for_Bass_Guitar.html (http://www.frontiernet.net/~jff/af2b/Dual_Channel_Blackface-style_Preamp_for_Bass_Guitar.html)
-
I've attached a schematic for the Organ Mate Reverb I'm cannibalizing.
I like the Hoffman preamp idea that Sluckey suggested. I have all the resisters & caps on the BOM in my parts bin already. I'd be reusing the PT, rectifier, the heater wiring, the tube sockets, terminal strips, some wiring and the power switch. The box I'm using is almost exactly the same size as Hoffman's, and I'll have enough room without it being too cramped.
-
That Organ Mate Reverb PT will work well for this project. Hoffman's preamp is pretty high gain like the Marshall 2204. If you intend to connect this to the input of your 18W clone you'll likely get a real squealer.
-
Wow, perfect! I didn't realize it was based on the 2204 or that it's high gain. Maybe I'll skip the mic input.
-
The Hoffman circuit wants 3 x 22mfd filter caps. I have one 22mfd, a 30mfd, and two 40mfd caps. I understand raising filter cap values too much can have a detrimental effect, but I'm trying not to spend money on this project! Will 92mfd of filtering vs the original 66mfd be too much? If it seems OK, is there an accepted order for the 3 different values? I understand the lead cap is key in a full amp as it filters the power tubes, but does it matter in this case? Thanks!
-
a pre has no sag, the only consideration, are you keeping the tube rectifier or going SS (diodes)? if you're keeping the tube, look up the datasheet and see what it wants for a Cap after the tube, typically 33uf is about max for some.
If you're doing diodes, filter the crap outta it :icon_biggrin:
-
I'll be using diodes. Do you get sag with preamp tubes? Anyway, I'll go with the caps I already have. Thanks!
-
a pre has no sag,
the 2 tubes are drawing, what, 10mA max?
I guessing if you undersize the PS enough.......
-
Sorry, my reading comprehension has gone all to hell lately!
-
:l2:
I got 62yrs of documented cases :icon_biggrin:
-
OK, I got the preamp constructed, brought it up slowly, no magic smoke anywhere (phew). I'm currently troubleshooting. Everything seems stable, though I can't check the B+ until I get new batteries for my meter. It's been displaying weirdly for days, and today it said the B+ was 460v to ground (with the tubes installed), so I hope it's the batteries.
One part I left out is the 47ohm R16 resistor going to ground from the heater wiring. I've never had one on any of my amps, so I'm guessing it has to do with the primary of the reverb transformer being connected to the heater circuit. Is it necessary?
Second, there are two spots on the schematic that are unclear to someone of my skills. I've marked them on the schematic copy attached to this message. At first I assumed they were intended as "jumps", not connections. Based on looking at the photos on the site, I've decided these intersections are actual connections. Do you guys agree?
-
yup, should be a dot there
-
No dots on the schematic. These are the kinds of things that more experienced people just know, and I imagine this project was directed toward people more knowledgable than myself. Anyway, I did end up wiring it correctly.
It just occurred to me that the ~360v B+ measurements might be real, not a symptom of failing batteries. I used the two 4.7k 2 watt resistors between the filter caps from Hoffman's circuit, rather than the 15k resistor called for on the original Organ Mate schematic. Looks like a 1k, then a 4.7k, then a 15k. It did have 4 filter caps tho. Anyway, that's a lot more resistance than the 9.4k total the way I built it. Is that why my voltages are so high? Should I trade out the second 4.7k for a higher value?
-
> 47ohm R16 resistor going to ground from the heater wiring.
That's a terrible drawing. There are TWO 47r resistors off the heater system. Both go to ground. But the drawing takes on to an existing ground symbol. Save the draftsman 30 seconds and made decades of confusion.
-
I have been told by more than one person that the schematic is terrible! I feel better about abandoning the original project. It was my first true point-to-point build, and all I had was that schematic. Ugh.
-
So what output transformer were u using in the reverb? I would guess you need maybe 25k load? This is probably wrong, havent seen a reverb driver like that before.
-
No idea, a small unmarked transformer from the original Organ Mate. I thought of it as a reverb transformer, like the ones in Fender reverb amps. Same thing?
-
The RT impedance could be 5k to 50k. 50k costs more (too much fine wire). Best power might be 15k. But this is not a monster spring shaker. Or a precision hi-bucks build.
The 15k-25k transformer in Fenders is really a stock part for communications radios with 300V supply for good RF/IF overload but only about 1 Watt of audio power because they don't have to fill a room, and the output is often all static. Run with a pentode that's a bit over 1 Watt. Run with a good triode it's almost a half watt. In this case the "power" part of the 12AU7 has little gain in front so it may not even do that much. A 5k might make more grunt with available signal, and may be a little more common. But if you find-out this is really a 2k transformer from a 50C5 table radio, I won't be shocked. It's clearly not Hammond Church Organ grade, but "affordable family fun" grade.
-
I have to share this embarrassing mistake. A few days before I dove into making this pedal, I did a bit of advance work, and the very first task I tackled was making the shielded input lead and soldering it to the input jack. Apparently I absent-mindedly wired it backwards. I screwed up the first step of the project, and then missed it over and over because it was too simple to double check. I'd been unable to get any signal through the circuit (obviously) since I finished the build. I finally noticed the reversed lead after a day of troubleshooting. Once I corrected that, everything worked fine. My lessons here are: A. don't drink IPA and solder, even if it doesn't involve dangerous voltages, and B. check everything, even the really stupid, basic stuff.
Thank you all for your help and suggestions!
-
I think I need a power amp to go with my preamp. I have the iron, sockets, and a few parts from a '50s/'60s Grommes mono PP 6V6 amp lying around, and the attached Dynaco schematic looks achievable. I take it this schematic was meant to show the power amp section of an integrated amp. Is there any reason why this circuit wouldn't work as a freestanding power amp? I'd like to build it into the same Bud brick-sized project box that I used for the pre.