Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Opry Audio on August 02, 2019, 05:35:58 pm
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Hey y’all! A customer brought me a Peavey classic 30 he’s put about $700 into already with a shorted PT and significant damage to the circuit board. There’s so much freaking room in this chassis, building just about anything into it seems possible, but I’m wondering about the tube an knob configuration being close together. Has anyone built anything cool into these chassis’? He’s interested in a brown face deluxe with trem which I will happily oblige! :)
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save $3 and keep the 4 84's +OT, and build this :dontknow:
https://el34world.com/Hoffman/files/Hoffman_AC30.pdf
otherwise anything BUT the pcb, save the tech world, recycle :icon_biggrin:
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Few years go.. someone gave me this non-working Peavey. As you know , the cabinets and chassis used to be nice.
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So... I gutted it , made a 1-12 baffle instead of 2-12 , and built the usual AB763 with reverb in it . The Classic 30 may be a good size for a single channel AB763.
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save $3 and keep the 4 84's +OT, and build this :dontknow:
https://el34world.com/Hoffman/files/Hoffman_AC30.pdf
otherwise anything BUT the pcb, save the tech world, recycle :icon_biggrin:
I think I’m going to go with the AC30 build! Does anyone have suggestions for PT that might be good? I don’t want to weigh down the amp too much. Would it be worth it to look into using a 275-0-275 transformer and a separate filament transformer? Anything that would work for an AC30 on the smaller side? Rectifier winding is not essential :)
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The PV C30 PT is a dog (along with the layout and the PCB), so you'd definitely want to change it.
The HT is designed to deliver a plate voltage of 300V, with a bias voltage source of -17VDC or so (which you can then reduce according to taste). So if you like the voltages in the C30, then aim for this as your guide when looking at a new PT. I had one made a little while back when I was thinking of doing this.
Rated for about 85V.A
Winding specs
HT = 0-260VAC @ 200mA (for FW bridge)
Heater = 6.3 VAC with centre-tap @ 5A
Bias = 0-15VAC
The other thing to bear in mind if re-using the stock chassis, is that the pots PV uses are non-standard sizes, and are smaller than 'normal' CTS guitar amp pots, so you need to drill out the pot mounting holes to bigger diameter, and think a bit about how you're going lay the chassis out - but otherwise there's a decent amount of room in there for a PTP or eyelet board etc
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The PV C30 PT is a dog (along with the layout and the PCB), so you'd definitely want to change it.
The HT is designed to deliver a plate voltage of 300V, with a bias voltage source of -17VDC or so (which you can then reduce according to taste). So if you like the voltages in the C30, then aim for this as your guide when looking at a new PT. I had one made a little while back when I was thinking of doing this.
Rated for about 85V.A
Winding specs
HT = 0-260VAC @ 200mA (for FW bridge)
Heater = 6.3 VAC with centre-tap @ 5A
Bias = 0-15VAC
The other thing to bear in mind if re-using the stock chassis, is that the pots PV uses are non-standard sizes, and are smaller than 'normal' CTS guitar amp pots, so you need to drill out the pot mounting holes to bigger diameter, and think a bit about how you're going lay the chassis out - but otherwise there's a decent amount of room in there for a PTP or eyelet board etc
The only part of the amp that’s staying in tact is the OT and tubes! As far as voltages go, I’m not married to anything about the classic 30.. imo I don’t think they sound that great anyway. No offense to Peavey.
I’m not sure if I’m even looking for a bias supply either. My plan is to do a PTP layout of the Hoffman AC30 with some additional requests from the client: an external speaker jack and to retain the effects loop which might end up being a dumble style effects loop if I end up doing normal channel only and free up a tube.
Thanks for the tip!! I’m gonna see if there’s anything that currently fits my needs. Is there a specific reason it would be advantageous to use a -15v bias supply instead of cathode bias? Aside from it being fixed
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You don't need a fixed bias supply for an AC-30.
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So, assuming I'm going with a solid state rectifier, and only needing a HT and Filament supply, is there anything out there that would be a good candidate? I can't find anything on hammond's site that's exactly what I'm looking for.
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Hammond 290NX or 290MX are replacements for Vox AC-30. The NX has a lower voltage.
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The build is going pretty well so far. Tube sockets and transformer are mounted! I went with the Hammond 290MX
I have to come to a decision about the faceplate layout in the next few days. I'm probably going to try and do a normal and top boost channel from one input and make it switchable between either of them with the option of jumpering them. I could use some advice for the most economic way to make that happen. Has anyone tried this before? It would be nice to make it footswitchable as well.
I'm pretty happy that there's enough room to spare for all this stuff. The client asked me to put in an effects loop as well, so I think I'm going to go with a mojotone unit. Has anyone had experience with these? Do you think I'd be better off doing a normal channel AC30 and doing a dumble style effects loop with the spare 12ax7?
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Re the AC30, I take it you are doing a 1-channel pre-amp (given you seem to have 3 pre-amp tube sockets to works with)? At about this point, it would be handy to see a schematic.
There are various ways you could add an FX loop - If you are doing a simple preamp with input stage, tone stack, recovery stage and LTP, you have a whole tube left over for FX. See attached schematic for different FX loop choices using 1 x 12AX7.
Or you could keep the spare tube for a 1-channel reverb (see the pre-amp in the attached SW22R schematic, which works very well for a 1-tube reverb) and attach one of those commercially available off-the-shelf modular FX-loop-on-a-PC-board setups (either tube or SS)
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I went with the mojotone effects loop. I do have a quick question about the Hammond 290MX, it has 2 primaries: is this only for wiring it for 240v? It probably doesn’t matter which one you use correct?
Also, there is a 3v center winding for the heater supply. That isn’t a center Center Tap is it? Using the 6.3v and 0v supply should provide 6.3v but not sure what the 3v supply is for. Thanks!
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I went with the mojotone effects loop. I do have a quick question about the Hammond 290MX, it has 2 primaries: is this only for wiring it for 240v? It probably doesn’t matter which one you use correct?
You may want to contact Hammond about that. It's not specified in the data sheet. Many times when a transformer has dual windings you will connect the two winding in parallel for 120V operation or in series for 240V operation.
Also, there is a 3v center winding for the heater supply. That isn’t a center Center Tap is it? Using the 6.3v and 0v supply should provide 6.3v but not sure what the 3v supply is for. Thanks!
That green wire is a center tap. Connect to chassis.
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the winding diagram is on the top of the transformer. Looks like two 120v primaries. Like Sluckey suggested, youll need to wire them up in parallel, which needs to be done to hammonds guide here: http://www.hammondmfg.com/5CHook.htm (http://www.hammondmfg.com/5CHook.htm)
Phasing will matter here. Just follow the diagram.
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> not specified in the data sheet
Two 120V windings, with implied polarity (0 and 120, and dots). Seems it would be clear to any experienced transformer smoker.
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This amp is almost complete, everything looks good except for the heater supply. Upon initial startup I was getting a supply down the entire line. I put a set of tubes in there and I believe there was a short to ground in the heater supply. When I brought the voltage up the indicator light is no longer lighting and there was a present 60hz hum. I need to see if maybe one of the output tubes was shorted. Could the transformer survive something like this? I read continuity to ground along the entire line, but I’m not sure if it could be the center tap? I also need to confirm that the heater winding is still good when I’m back in the shop tomorrow. Thanks to everyone who’s given me input on this
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read continuity to ground along the entire line
power down, wait, take out tubes, lift CT of filament, if you have one, or made one.
OHM across the filament pins of the pa tubes, then ohm 1 pin to ground, then the other pin to ground
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After removing the center tap there isn’t a short to ground! That’s good, maybe the problem lies somewhere else? No continuity between the 6.3v pins and ground either
What would be a good next move?
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What would be a good next move?
with CT taped off, all tubes out;
gator clip a meter across the filaments, set to read VAC.
2nd meter, gator-clip from ground to the 1st B+ tap, other lead to ground.
power on, read, power off.
(One meter, you'll have to do it twice, start with the B+ tap)
power should only be on for seconds, NOT minutes
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The filaments are all good, I found a miswiring in the PI, wasn’t grounded.
Now I have signal but low output— at least that’s something better to work from. I signal traced up to the end of the PI and all looks good, plenty of gain!
So there’s something wrong with the output stage, one side of the EL84’s is redplating like crazy. All voltages to the plates are the same. I haven’t run it at full power but I got something like 275v from 70vac on my variac. All voltages look like they’re dropping like they should!
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redplating like crazy
we haven't got there yet, you cheated!
here's what I'm looking at;
https://el34world.com/charts/Schematics/files/Peavey/Peavey_classic30.pdf
with the PA tubes OUT, the preamp tubes OUT
power-ed up, does it blow a fuse?
what is your bias VDC
the 15 VDC
the 1st tap B+
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B+ voltage unloaded is at 340v
Not sure what you mean by 15vDC can you clarify?
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No blown fuses! Just to be clear I built the Hoffman AC30, so it’s quite a bit different from a Peavey Classic 30, maybe the same in the output section
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Oh! There we go, no voltage on the cathode resistor. But is it because I have the tubes pulled?
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Just to be clear
found the line, i'll go find schematics n study
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this one?
https://el34world.com/Hoffman/files/Hoffman_AC30.pdf
If so; with no power no tubes ohm R34
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Yes exactly!
I’m reading 50ohms along pin 3 of each tube to ground, so it’s all connected. In theory it should be pulled to ground.
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when you brought up power did the led light?
did you start with new PA tubes or the old ones from your peavey?
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Yes, in fact, the amp started putting out full volume. It’s super loud now, the pilot light and heaters are working properly.
I’m getting full output now too. I put in a set of known good el84’s but 2 of these might have gotten burned up. Better than the PT.. but anyway, the bias voltage on the cathode resistor is 7.5 but it should be a bit higher, no?
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The amp in general is very noisy. Lots of little gremlins and pops to find.. but the output tubes are particularly very sensitive. I think there’s some low frequency oscillation coming from somewhere too
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Seems it would be clear to any experienced transformer smoker.
I found a miswiring in the PI, wasn’t grounded.
what happened to the filament resistors?
my guess;
the PI floating possibly sent your 84's into fixed bias?
If you wired R's to the filaments and connected the CT......
I would leave 7.5 there for now, it's loud, fix the soldering wiring gremlins
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I got most of the gremlins sorted out. I am now having a problem with the mojotone effects loop. When the return trim is all the way down, it goes into high frequency oscillation.
Upon my last startup, none of the tubes lit up. Heater voltage was being supplied just fine, but no voltage on the cathode resistor. I might chalk it up to that these tubes are done until I get new ones to know for sure
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In fact, no cathode voltages anywhere, or barely any :w2:
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the amp started putting out full volume. It’s super loud now
something changed from above to below, can't help much there, wasn't there, got witnesses :icon_biggrin:
In fact, no cathode voltages anywhere, or barely any
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Unfortunately no witnesses... but if it indicates anything, the output tubes did light up, it took abnormally long to do so. And only had output for about a minute before it went back into nothingness
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The bulb is still working just fine
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1/2 split
install the preamp tubes, leave out PI and PA tubes, make filaments glow, cathodes get volts, sounds like you have a scope or listening amp, make a signal go from input jack to last Pre-amp tube output.
Once that's good as gravy
make other 1/2 behave :laugh:
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What could the problem be potentially? Could it be that there’s no heater current? I don’t think it was shorted to cathode or anything. Still nothing conducting from the preamps :( anything I should try?
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Nope about 600ma in the heater supply
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600mA draw, as in load?
anyway;
you had heaters
you had sound
you did something
you have no.......
always start there
then start at beginning
no tubes, make fil work, if that means taking out whole string....
You have yet to address;
did the filament R's burn up?
did you have both CT AND R's connected?
not a big deal, we've ALL smoked :icon_biggrin:
just trying to see what makes sense
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600mA draw, as in load?
anyway;
you had heaters
you had sound
you did something
you have no.......
always start there
then start at beginning
no tubes, make fil work, if that means taking out whole string....
You have yet to address;
did the filament R's burn up?
did you have both CT AND R's connected?
not a big deal, we've ALL smoked :icon_biggrin:
just trying to see what makes sense
Yes, CT is connected. I read .6A from the pilot light to V1 with output tubes and PI pulled
Something did seem a bit odd though. I read about 50R between the primary and secondary on all tabs except for the rectifier, and 50R between the heaters and HT winding. There’s definitely a short somewhere right?
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definitely
yup, expect normal ~~~~thousands of ohms charging to many more K ohms
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What’s the best way to check if the transformers been damaged? If there’s continuity between primary and secondary it’s basically *smoked* right?
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right
be my guess
I'm nuts enough to take PT out of amp onto bench
gatorclip my meter (the cheap one:) to the secondary set for VAC
put 2 fuses INline with my power cord, into my breaker buss strip n see if the lights flash. NOT recommended :icon_biggrin:
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I should check if any of my power supply survived.. what a nightmare. Would a failure like this not blow the mains fuse?
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Can you post any hi-res gutshots on your build? (Ideally you need a camera with a macro-zoom function to get close enough without being all blurry, or you need a high-resolution setting on your camera - which will be high-megs)
If so, maybe we can spot something
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Just a quick update— I was totally mistaken about the PT, the short I was reading was from the CT for heaters and HT and the mains ground. I touched up some of the connections on the tube pins and I have regained heaters! They work now, so happy!!
Present issues currently are idle hum which may be coming from the power transformer. The heaters are pretty out of the way. The PT is getting pretty dang hot. No smells or cracking or anything like that just hot.
There’s a mojotone effects loop installed which is giving me some issues. The customer requested that I put a return trim control on the faceplate which I did, and it kind of works except it also trims the dry signal. And there’s a lot of noise coming from it as well. But no DC going to the send.
Lastly, I’m having a problem with the channel switching. Maybe I need to find an alternative method. What I did was wired the input jack to a 3 way selector switch for switching from normal to top boost and both from one input jack. There is a considerable amount of bleed from each channel. i.e. when the top boost volume is up and the switch is on the Normal channel, there’s a nasty hum present and vice versa. Any suggestions on how to make this idea better? I used a pickup selector switch since it was the only full size switch that would fit between the pots and the back of the chassis but I may need to move it to put in a Carling or something like that
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I believe in addition to wiring issues, you have grounding connection issues. Check the solder joints and please tell us what sort of grounding scheme you used. And as requested- Pictures of the input jack wiring would be helpful as well as the build in general.
How are your soldering skills?
silverfox.
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Here are some close up pics of the build
https://ibb.co/6FH6f2H
https://ibb.co/rdFfzC8
https://ibb.co/xf7YvZK
https://ibb.co/NsQhP9z
https://ibb.co/PxBJNZd
https://ibb.co/DtgFKCk
https://ibb.co/zxvVhDY
https://ibb.co/6DcH81f
https://ibb.co/KDvk06Q
https://ibb.co/qRJd4DS
Yes there definitely are some wiring or grounding issues. My soldering skills are good (at least I think so!)— I used a bus ground soldered to the pots and input jack. I grabbed a separate ground for the 220k resistors just before the output stage.
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Hey y’all! A customer brought me a Peavey classic 30
You have customers? That's scary!
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Hey y’all! A customer brought me a Peavey classic 30
You have customers? That's scary!
Does this amp really look that bad? :(
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Hey y’all! A customer brought me a Peavey classic 30
You have customers? That's scary!
Does this amp really look that bad? :(
No. The fact that YOU have customers is scary!
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Hey y’all! A customer brought me a Peavey classic 30
You have customers? That's scary!
Does this amp really look that bad? :(
No. The fact that YOU have customers is scary!
Oh! If I didn’t work out of a brick and mortar guitar shop I probably would be f****d
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Now.. if you have any constructive criticism or direction on the build... I’m all ears Sluckey :BangHead:
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Hey y’all! A customer brought me a Peavey classic 30
You have customers? That's scary!
Does this amp really look that bad? :(
No. The fact that YOU have customers is scary!
Oh! If I didn’t work out of a brick and mortar guitar shop I probably would be f****d
No. The fact that you don't have a clue what you are doing means your customers are f**ked! Think about it. It's one thing to not know when you are a hobbyist. It's incompetence when you have customers.
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I got ADD, had a bud, all those letters messed up my country music :icon_biggrin:
could you label the one that shows full gut shot
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Hey y’all! A customer brought me a Peavey classic 30
You have customers? That's scary!
Does this amp really look that bad? :(
No. The fact that YOU have customers is scary!
Oh! If I didn’t work out of a brick and mortar guitar shop I probably would be f****d
No. The fact that you don't have a clue what you are doing means your customers are f**ked! Think about it. It's one thing to not know when you are a hobbyist. It's incompetence when you have customers.
Thanks for your input
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Sometimes it just needs to be said.
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Thanks for the pictures. Don't take this as an insult. Those solder joints look really bad. I'd watch some soldering videos on youtube. A simple search reveals many: youtube.com soldering videos.
Then I'd practice until I was able to produce a good soldering connection. Then I'd reflow all of the solder joints. The chassis connections will need a high capacity soldering iron to generate the heat needed for the connection.
At one time all of us here had to learn good soldering techniques. My first project was a Heath Kit solid state guitar amp. I used acid core solder and didn't know how to get good flow. Disaster. Never got it working and the circuit board corroded from the acid core solder. Next Heath Kit project: A 5 mhz Oscope. Turned out much better as I had gotten my first tech job and learned how to solder.
Now if I could just do the same with copper pipe.
silverfox.