Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on December 14, 2021, 03:50:04 pm
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Hello,
I've been rebuilding the amp for a friend. He did me a good turn and I offered to fix a couple of amps in return.
I understand that these amps aren't as loved as some, so I'm wondering if there are any mods that have been found that would be worth trying?
This amp had tweed disease - a damp and conductive set of circuit boards that you could measure voltages on. The amp whistled, tweeted and farted, sounding a bit lifeless too. It had been stored in a less than 100% dry environment judging by some of the rusty bits. From experience, its pointless trying to dry them out, especially here in the soggy south of England. A lot of gear gets stored in garages which can be less than ideal. It also had a few dodgy repairs.
So I've now got it fully working by replacing the all the circuit boards with fibreglass/eyelet ones that I made using the originals as templates. All voltages are correct and it is now super quiet to the point where it is hard to tell if its on.
I fixed a few faults along the way too:
- One of the output valves was cooked (burnt markings) The 470R screen resistor had been replaced with a 470k. Replaced it and all the others with new 470R/2W low noise Kiwame carbon films.
- On the same valve the 1k5 grid resistor had split in two. replaced with new carbon 1k5.
- Replaced the Sovtek 5881s with a new matched set. I'd would have liked to have used Svetlana Winged C 6L6s but the price is prohibitive!
- On the cap can PSU board, a 2k2 had actually increased in value to about 4k4, halving the anode voltages going to the pre-amp valves (I've NEVER seen that happen with a resistor, and the markings were correct for 2k2.)
- The hum balance pot had failed. Replaced.
- All electrolytics have been replaced, everything else is original.
- All the PSU board resistors have been replaced with Kiwame 2W and 5W low noise carbon film resistors (couldn't get the 2W ones in a couple of cases)
- All functions work: tone and volume controls, reverb, vibrato, and the Master Volume distortion thing.
- Output is around 90W. Due to the 5881s I guess.
- All voltages are as per the schematic, within around 10%
My friend loves the amp and has had it a long time, however the tweaker in me cant help wonder if there is anything I could consider in making any "improvements"?
This is what it looks like now -
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> a 2k2 had actually increased in value to about 4k4, halving the anode voltages going to the pre-amp
That would double the voltage DROP, not halve the voltage to.
Maybe I am too respectful, but I don't think "mods" help most older Fenders. Leo had it good when he sold, and CBS hadn't screwed-up too much by 1976. Buy some pedals.
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The pull boost is a unique sound, and does have its fans, my chum being amongst them :dontknow:
It is what it is, and probably best left alone. You’re never going to get Dumble type smooth overdrive out of it.
Dunno about refitting that ancient white cathode bypass ecap, even if it still tests ok?
Carbon based resistors in high energy applications (eg screen grid, HT droppers) aren’t a good choice these days, now the alternative of MO is available. Carbon provides a fuel load that can ignite and cause collateral damage if something shorts.
See Aiken’s technical q&a http://aikenamps.com/index.php/technical-q-a
Q:What kind of resistors do you recommend for power supply decoupling use?
A: I recommend 5W (or a higher power rating if needed) 1% 750V metal oxide resistors for this application. They have higher surge/overload capability than the film types, excellent long-term stability, and flameproof construction. Note that lower-power metal oxides, such as 1W and 2W are usually only rated for 350V and should be avoided for this application. Even though the voltage across the resistor in normal operation is only 50 volts or so, it can be much higher at turn on, or if a tube fails. Another good type to use is the Ohmite OX/OY series ceramic composition resistors. These don't have the tight temperature coefficient or tolerance (typically 10%) that metal oxides do, but they are designed for heavy surge use.
Q:What about screen grid resistors?
A: I recommend 5W 1% 750V metal oxide flameproof resistors for this application. The flameproof "cement box" resistors are also good, but are usually only available in 5% and 10%, and must be rated for at least 7W or 10W in order to get an appropriate voltage rating, because the 5W and lower units are usually only rated for 350V. In normal operation, the screen grid resistor only has a few volts dropped across it, so a 350V resistor is overkill. However, in the event of an internal tube short, the voltage drop can be much higher, up to the level of the supply, and the current through the screen resistor will increase dramatically, increasing the voltage drop across it. Normally, the fuse will blow fairly quickly, but sometimes not before the screen resistor is toasted. There are two possible failure modes, over-voltage and over-dissipation due to the increased current from the short. If the voltage rating is too low, the resistor element will flash over and either burn up or carbon track and change value, leading to eventual failure down the road. If the voltage rating is made higher than the supply voltage, there is only one possible failure mode of the resistor, over-dissipation. The metal oxide flameproofs or OX/OY ceramics can usually take a higher-than-rated surge for a short time before they burn up. If the fuse is appropriately rated, it will blow before this over-dissipation can harm the resistor. A quick fuse and tube swap and the amp is back in business, instead of having to replace the screen resistors.
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VoxBox
Nice job on the Fiberglass replacement board, esp the Dogbox.
+1 with pdf64. Considering the stability, performance and reliability of MO (Metal Oxide), I will always use MO in all Tube high power applications.
Regards
Mirek
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Thanks for the compliment Glass54 :icon_biggrin: and Thanks PDF, knew I'd read that somewhere. Thanks for the reminder. :icon_biggrin:
Take your point about MOs, but some folks have a thing about the old carbon Rs, sound wise. I used the Kiwames as they are low noise albeit Carbon Film, and don't affect tonal sound quality as they are in the PSU, but could affect noise performance. Its outstandingly quiet! Unfortunately I don't have a working noise meter to do a proper test. (Anybody got a schematic for a Soundex AM100 noise meter?)
I used to be torn between making something the "best" it could be and leaving well alone as the act of actually going in and changing something can affect reliability. Especially on something that has been untouched for almost 50 years. I've come down on the side of "if it aint broke, don't "fix" it.
When I was a newbie engineer at the BBC, working on analogue studio equipment, one shift used to tweak everything to its "best" the other shift left well alone if it was good enough. The shifts used to change every 12 months or something like that. Guess which shift had the most faults.....
BTW I did tests on Carbon vs MO for noise some time ago. Carbon noise sounds like SSSSHHHHHHH. MO noise sounds like ssssssssssss. I'm not getting into one sounding different to the other!
Thanks again everyone :icon_biggrin:
Cheers, VB
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The carbon resistors I mentioned were specifically HT droppers and screen grid current limiters.
Unless the HT decoupling caps were bad, any noise generated by the HT droppers should get shunted to 0V common.
And it doesn't seem feasible for the screen grid current limiters to generate enough noise to have the faintest affect on the signal?
Other than a theoretical increase in noise, there's no issue with carbon resistors elsewhere, it's just where a reasonably foreseeable potential fault could cause massive overdissipation and, with a fuel load, result in ignition and collateral damage.
Kudos for having time at the BBC on your CV, please tell more! :icon_biggrin: