As this is tech-talk i thought some of you might get a kick out of this one.



After some debate a friend of mine sold me this rather scarse old Hummel amp. REALLY run into the ground,and my friend saved it from becoming parts of a scrap pile about 8yrs ago and it´s been standing around since. Dating this thing is rather simple after all as the entry tube is an ECC-808 and as this was put in the market back in -62/-63 that kind of pinpoints the year of manufacture as i see it. I DO stand to be corrected on that point though.
The ECC-808 then being used by Klemt,Telefunken,Hohner and others as it was introduced. A great tube IMO,albeit expensive these days.

One could easily get fooled that the amp is bigger than it is,but suffice it to say that this thing is about on par with a Klemt S-40/BS-40 approx.



Within what greets us is the choice of a rather odd powertube,the 7355. As the amp is as low in profile as it is these are installed tilted to make room.
That the amp sports 7355´s tell that it isn´t impossible that the main intent of the amp was the USA as that powertube isn´t especially common here in Europe. The 7355´s are about moronic to use today and for the better they are about deadringers to a way more common tube,the 7591. Ergo,this thread is about swapping powertube type,amongst other trix. However the modern day n era 7591 is a taller tube than 7355 and thus this needs to be handled. Space really is a scarse commodity in this case.
Thus an entirely fresh "spacer plate" will be fabbed to make this possible.



From a purely electrical point of view no real groundbreaking news to report,if we disregard the amps age. The tremolo circuit for instance is basically a deadringer for what we find in a common Fender Deluxe reverb. A tagboard,and this tagboard is rather heavy duty. "Mustard" caps used throughout,with few exceptions. However...amp had been run into the ground alright and in turn some "Mr Handyman" has pulled some repairs to it why the rectifier setup for bias voltage,choice of wire dimensions and what not...geez... heavy duty enough to work as a god damn tow line! So,replaced those bits right off the bat.

Measured ESR for the e-lytes and let her rip. In a few minutes the B+ fuse went sky high and this was traced to the first hit e-lyte that had gone belly up. Replaced that and the B+ fuse and hit it again and...victory. Amp was now up and running,however in real damaged form as it turned out.



In a rather typical german manner all things in due place and order. Brings that even those trim rings are screwed in place,and this with M2,5 brass screws 
That beautiful faceplate had seen better day though why i attacked it with some 400grit wet emery,water and detergent. It worked out just fine.



As i had no idea where this´d end up i opted to pick the VERY worn pots apart,retension them and hand them some general service. As was the amp scratched more than needed when one turned them pots so..



An adapter plate was cut to size. As it turned out i came to install the powertube sockets the wrong way around which made the looks of that thing come out rather clustered. Well,then we now that i guess.
As it turns out though the amp was in a more sorry state than i had believed why...this was going to need some more in depth help. The GOOD part though was that the adapter plate and the 7591´s worked like clockwork!



On the other hand..not to excited about the looks of THAT. That there needs to be handled,no doubt.
As you can see i unbolted the output transformer and as it turns out there was yet another tap in there. However...at the rear of the sheet metal housing was an old Dymo strip telling that this amp was setup the same way many Fenders are. Ie; "8 Ohms minimum" as far as the OT. The wire gauge used for the secondary side of it is absolutely insane!



For some reason there was a "sag" resistor of approx 20 Ohms in there. No idea as to why. Approx as...i have no IDEA what that there resistor has been through. It sure has led a hard life somewhere down the line.
As you can see even the anodizing of the chassis had charred,which certainly isn´t a good thing! That we see soot remains is a BAD thing.



This then the reason for this mayhem. Somewhere down the line there´s been a rather severe onboard fire The tagboard is REAL heavy duty why that damage there tells that the fire in case has been as well. Someone most likely replaced a fuse with the wrong rating one and the rest as the say is...history.
That kind of settles it though. I opted to lift the entire stock tagboard out of there and run the entire 9 yards. No point in trying to beat life into a dead horse and this way the actual amp will be saved for future generations,and i in turn can set the amp up as i see fit.

B+ came all in at approx 480VDC@approx 30mA per tube. In other words rather high voltages seing the 7591´s.

So. How does the stocker sound? Not all that to be honest. Now...of course i pass that judgement on the amp in damaged form,which should be admitted,but at least a working such.
Cleans of it is fair i´d say,but that´s about it. The phaseinverter is a floating paraphase and...as it turns out that choice was the WORST of two worlds because when pushed into distortion the amp changes face to something downright ugly.

Well. Turretboards,fresh capacitance and what not had been picked up from TubeTown so...at it.

Well,now it might be more evident why i opted to pick the entire thing apart. That fire had been rather extensive. That soot and what not turned out to be downright impossible to get out of there however..



Yeah well...



Out rear it had even come to rust around the voltage selector.  So i went at it with the rotary wirebrush in the diegrinder...and cleaned the thing out completely.



...and came to spray the thing silver. This was now going to turn into a full 9 yards resto/mod. The paint of course bringing that i had to clean the chassis contact points out well.



Ah! German "order". I like that. Secondary wiring of the power transformer had been damaged in the fire to the point where the wiring needed to be replaced.
Said n done.



New is new after all so out rear a fresh IEC jack,a fresh pair of fuse holders-one for mains and one for B+...and the stock voltage selector in turn was cleaned out.



Yep. Fresh tube sockets too. All the stock parts saved in a plastic bag.



A fresh "bias board" was put together. Individual adjust of bias voltage,and this for a reason. 7591´s ain´t exactly the most common tube to be had in quartets why...this is a simple way of handling that. As you can see the main HT rectifier was replaced as well.



Still needs to be tidied up a bit,but you get the general idea at least.
Fuse holders for the amps two heater winding for looks reasons placed beneath the bias board in turn.



Yep. Even fresh mains wiring. No stone left unturned. There in the pic btw...another small trick. Putting together wire bundles,to keep wiring tidy,i often use small pieces of shrink sleeve. Works like a charm



Yep. TubeTown turret board. I use these a LOT..good quality stuff IMO.
To me there´s few things as boring as putting fresh heater wiring down when i do a new build. I absolutely LOATHE doing that,the more reason to get it right the first time out.



Heater wiring. The one for the preamp of course with a 500 Ohm "humdinger" trimpot.



That feels tidy enough i guess. That there is the goods for the entire preamp and phaseinverter.



This then is just cool beyond words! Rather scratched and full of battle scars so i yet again took some 400 to it..better but still not done. Then again..the amp IS from 1963 some time..i guess.
The input jacks i replaced with fresh Cliff ones as I want to be the one to tell where the entire thing hits ground thank you very much.
The "presence" switch,which in reality is a bright switch,will get new tasks in life. Channel select to be more specific.



This is just to cool. Please be adviced that someone cut that there out in the aluminium in an era way before CNC was even an itch.
One singular 7V lightbulb won´t do it though. I want that there "Hulk green" to light up as a christmas tree all said and done...



New is new after all. Stock the amp uses long shaft pots but...who wants to sit around awaiting delivery of that? So i cut some rubber hose to size...as it turned out though..i opted for a different solution all said and done.

As i´ve got a lathe around i opted to turn these instead out of aluminium. Felt way more secure.



Yep. The powertube sockets got turned the other way around alright. That piece of turretboard there in turn holds the individual capacitors for the powertube g1 points. Center of it in turn will be populated with the components for the ECC-82 run effects loop.



...and as you can see things are coming along.



Out rear...the jacks and pots for the loops send and return. A bit worried about the real close proximity of the speaker jacks to the send jack..



Running out of room. I really would have liked to install that footswitch jack for channel control in the already existing hole but..the adapter plate for the 7591´s made that impossible. Well..can´t win ´em all i guess...



Uhu. The "AC side" of the amp is getting cramped for space. On the other hand all that needs to be in there is...now. So no harm no foul i guess.



Wiring for the PPIMV was laid down. Opted to use shielded wiring seing the direct proximity to HT wiring. A bit unconventional i guess.



Lower switch is the stock mains breaker. The upper stand-by one had been replaced by this rather ugly looking half assed chrome thingy so i dived into my spares and dug out an at least reasonably alike looking switch. Details? Might be...still..



Shod the edge of the cutout for the powertubes with some plastic piping.



Opted to install an extra light bulb for that Hummel insignia. Nah..that won´t cut it so..LED´s...here i come.

Amp is dual channel now by the way. Controlled by H11 optos. These optos in turn are run by a small switched 5VDC PCB transformer and the whole thing works very very well.



..and here we go. Net result,from an electrical point of view at least. That quartet of 7591´s doing its thing and doing it rather loud! I haven´t measured output yet but...powerful the old amp certainly is.
Dual channel as stated and in lead mode the amp is downright insane! 4 gainstages heading into that ECC-82 for the loop and from there into a longtailed pair phaseinverter.

Lesson learned is that it can often pay off to investigate the concept of replacing powertubes when we´re talking oddball ones. The 7591´s i shoved in there is doing their thing like nobodys business and...will keep on doing so for a long time i hope.

After tossing a few tweaks at this thing it turned out to be one rather serious piece of kit. In lead mode it´s overtone prone it isn´t even funny.
Still haven´t measured output power but..it´s a loud customer alright! REALLY loud.

Cool restoration project!
Congrats :topjob: