Stromberg Carlson AU57 Amplifier Assessment What Work Does it Need? What Work Has Been Done?
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- Опубліковано 1 жов 2024
- Stromberg Carlson AU57 Amplifier. We take a look inside to see what work has been done this amp and to see what work is still needed to make this amp good again
It's my amp. When it was posted to me, it was in a single layer thin cardboard box - and that's what bust the on/off Treble pot knob (hence the replacement chicken-head knob).
The power transformer was changed to one that offered 240V taps, so I could use it in the UK without a step-down transformer.
Also, that big metal thing was loose on arrival - so I asked the seller about that - and he said "don't worry about that, it's not used any more - it's just there to look the part". I use it for guitar and hopefully harmonica. It's a monster of an amp. It worked fine until I took it to a local guy to check it over - then it had faults from that day since. Sounded awesome before :
ua-cam.com/video/IW949gCm9Fg/v-deo.html
I just watched your video. Holy $hit dude 😎🎸🎶
@snakehips81
Have no fear!.
Martyn will have this gem singing again, he did a Masco ME-27 for me last month and now it's probably the best amp I've ever had!.
One of the best techs on the Island.
According to the schematic, the 6L6 plates should be at 420 volts and the screens at 335V, so something definitely isnt right. The 5U4 supplies the 6L6 plates *ONLY*; the 6x4 rectifier feeds the screens, as well as the plates of all of the preamp tubes and the phase invertor tube. Perhaps something is dragging down the plate supply? I have to wonder if an output tube was inserted into the socket incorrectly, damaging a tube or component (hopefully not the output transformer). Or perhaps the replacement power transformer doesnt have sufficient AC voltage output (might need a change to solid state rectification to increase the B+). Note that youll need TWO fuses if you wsnt to fuse the B+ (one for the 5U4 output and one for the 6X4 output).
I was have just been studying the schematic. Isn't there a separate winding for the 425v supply. That would have been lost in the old transformer. So now the B+ supply voltage is all off the 5U4. The choke does not have enough drop for the screens. It wouldn't need to have much drop in the original circuit. No longer the correct choke? Screen resistors should reduce it below the plate. s3.amazonaws.com/tubedepot-com-production/spree/attached_files/40-18089.pdf?1487783171 Data sheet for the Classic tone. 312v AC @ 150ma. There is a lot of tubes drawing on that. 440 volts DC unloaded? I would have gone for 200 ma.
UPDATE: I looked at the schematic and realized that the original transformer had 2 high voltage secondaries: one for the 5U4 rectifier, which fed the 6L6 plates only, and the other secondary at a lower voltage for the 6X4 rectifier and the plates of all the preamp tubes and the phase inverter. The replacement transformer presumably doesn't have two high voltage secondary's at different voltages, and so the power supply is going to require some rework. I suppose the easy way would be to put a GZ34/5AR4 in the unit and probably raise the 6L6 plates by 30 to 40 volts; or use a solid state rectifier just for the 6L6 plates, to raise the voltage even further. This amp probably shouldn't be using metal 6L6's either, especially if you do rework the power supply to step up the voltage. Also, what is the current rating for the 6V AC secondary on the Classic Tone transformer? It might be somewhat overloaded with the 6X4 added onto it, as well as the 6L6's and 5) 12A-- tubes.....
@@vintageguitaramp_guitarteacher , ps, your comment just popped up now, *3 days later*, immediately after I posted my second comment here!
You could also feed the mains AC to the 220VAC input instead of the 240 winding; that would increase the overall B+; but you'd have to be careful that you don't exceed the 6.3 Volt heaters by too much. It's worth trying! Anyway, whoever selected this replacement transformer was an idiot......
Two rectifiers! I'm excited to see how this works out.
The amp does go very loud - but I tamed it a fair bit by putting that 12AY7 into the 12AX7 socket (in the 2nd preamp stage).
If you put a 12AX7 back in, you will find a big jump in volume (at least WHEN it was working , a few years back !)
LCR caps are made in Wales. I've never had a problem with them but maybe I've been lucky?
Hello. That's good info. Only one of the three in this amp is bad. Probably a one off. Another guy left a comment with some good info but it has disappeared. Comments just seem to that sometimes. Thanks for watching and take care
Is the speaker impedance matched ok, might account for the moderate volume?
Hello rob. It's been to another tech and it has not been right since. It's not a miss match on the speakers. Take care Rob.
What's with all of those 12AU7's? Does the schematic show all of them? From what I understand a 12AU7 is a driver tube, similar to a 12AT7, but unlike 12AX7, 12AY7 or 5751's which are used for amplification with different gain factors.
The schematic shows four 12AU7's and two 12AX7's. There is a later version of this amp that used mostly 12AX7's.
Hello Michael. They can be used for amplification. They only have a gain factor of 20 though. They are useful if you have a high gain amp and want to tame it. We will see how well they work in this amp once I get it running.
Take care Michael.
@@snakehips81 , the Sam's schematic I have for this model shows one 12AU7 for the phase inverter and the other preamp tubes are all 12AX7's. As for the 6L6 plate voltage being lower than the screen voltage on this amp, I bet that's because the power transformer was changed for a 220VAC transformer; looking at the schematic, the original transformer had an AC B+ tap to feed the 6X4 rectifier at a lower input AC voltage than the AC voltage to the 5U4 rectifier, but the replacement transformer doesn't have that additional B+ tap. Both rectifier tubes are presumably being fed from the same AC secondary, but the output of the 5U4 is sagging under the plate current load of the 6L6's, while the 6X4 only needs to deliver a tiny amount of current to the screens and preamp plates, hence no voltage sag to speak of....not to mention that the 6X4 is an independently heated rectifier tube and therefore has very low voltage drop.
@@goodun2974 - there are two versions of this Stromberg-Carlson AU-57 amp. My amp, in this video, is the earlier version. The tube type & arrangement is clearly printed on the back of the amp (shown near the start of the video) AND the schematic print-out, stuck to the inside of the chassis lid (actually, the bottom of the chassis !).
Your schematic will be for the later version of the AU-57 amp. I have a copy of the Sam’s “PhotoFact” document, which I sent to Martyn with the amp, but this document is for the later version of the amp - but I thought it might still be reasonably useful. The power transformer was replaced for me by the guy in the US that sold me the amp.
To my ears anyway, the amp sounded great - BEFORE I took it to a more local amp tech/builder company, just to check it over. It’s not been right since then !!!!
I know Martyn will be able to get it back on track.
Thanks for your input/advice.
Oh man I’m dying to mod this thing!! Who’s would use this as a PA? It’ll get way more use as a guitar amp.
Hello William. The customer has given me a bit leeway with this one. Part two should be out in two or three weeks. Take care.
I love the sound the amp had when it worked AND as far as I can tell, all 4 channels are fairly identical & should sound the same - so after discussion with Martyn, the idea is to keep at least one channel unchanged, but make some changes to the other channels, so the whole Stromberg amp has a different sound to each channel - for lots of flexibility. I can still try out different preamp valves in each channel, instead of 12AU7's.
Do you have any suggestions how to make each channel a bit different ?
I'm not after anything high-gain, just mild crunch that kicks in when you dig in and play hard. Kind of how it sounds already (in that video)
The first input is supposed to be switchable for use with a phonograph, so maybe it's a poor match for guitar, or it's been modified, or perhaps messed up. Mic 4 appears to have an impedance matching mic-input transformer plugged into the octal socket.
The first input worked correctly when I got the amp, including the 3-way switch - and the channel only went weird after I took the amp to an amp maker/repairer to have it checked over. Probably got a short or a wire knocked off somewhere !
Hello. I will have a close look at that when I start repairs. Take care.
@@vintageguitaramp_guitarteacher Ch1, with the 3-way switch and 12AX7, worked fine on the "Mag" setting of the 3-way switch - and sounded very similar to the great-sounding Ch2 & Ch3, when the amp was working.
I put the preamp valves for the amp in that cardboard box !!! (The plastic-lidded box had spare valves in it !)
Yes I found them. Take care.
Good to see this getting the attention it deserves 👍👍
V grateful for the recommendation to him.
Hello Ronan. Thank you for recommending me. Take care.
@@vintageguitaramp_guitarteacher if anyone can sort out the mess that's been left inside those amps, you're the guy. Plus, we hopefully get to see the process 👍
Service info for most Stromberg-Carlson amps can be found in Sam's Schematics. I'll take a look through my collection......(after watching the entire video, I see that you already have it).
Thanks mate. Take care.
The red wire is how the impedance is set, it's not a mod.
I'm sure the guys in the US will have a better knowledge of these, I believe these could be used with a turntable that hooked up to the A.C outlet in the back?.
Stromberg Carlson have loads of great amps for converting.
This should be a monster when it's sorted out!.
Hello Keiran. One or two of the American guys have chipped in with some info. Take care.