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I have a few questions for the pros...Have had great results from Enzo, Dave Mac Farlane, 52 Bill from previous projects and I repspect all of the help you have given people in their quests for reasoning..So Ill explain this story as short and to the point as I possibly can.Friend and fellow musician had a Vox Royal Guardsman head given to him and asked if we couldnt resurrect it the best we can,upon hooking it up to a cab it had very little and very fuzzed up output.So after replacing all the electros,diodes,driver and output trannies,new rca cable and a 3 prong plug it still gives the same result..as if nothing that was replaced made any difference in the output. I then connected the pre amp to the power amp of a Peavey Heritage and it sounded great through that,all 3 channels worked ,reverb and tremolo worked just as it should..Question is,what gives? Output transformer? I noticed a couple of the power resistors were showing off readings,maybe those? And as far as replacements,10 ohm 1w,180 ohm 2w,450 ohm 5w and .25 ohm 5 watt..I may be able to get the 10 and 180,but .25 and 450,Ive never seen a .25, what would the equilivents be for those? ok I just checked them again before posting this, the .25 and the 10 ohm power are showing no real resistance,as well as 2 4.7 and 2 6.8 ohm 1/2 watt..maybe those are the culprits.But any advice, suggestions would be greatly appreciated by anyone who has operated on this species before.
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Are you measuring those resistors in circuit? Most times you need to remove one leg out of circuit for an accurate reading. Also, resistors drift over the years and change their values. 10-20% tolerances are very typical margins for the parts.When the going gets weird... The weird turn pro! -
The good news is that you're past the hard part. Generally the preamp is what gives all the headaches. The power amp is unusual, but not difficult.
It's a driver transformer, not an output. It's the least likely thing to be wrong with the power amp, although like anything, they do fail, if rarely.Originally posted by dNi3L View Post
As Gonz noted, you'll have to lift one end to measure those. Low resistances are hard to measure anyway, but it's even worse trying to do it in-circuit.I noticed a couple of the power resistors were showing off readings,maybe those?
With some exceptions, the nearest standard values will be fine. But you can probably get what you want. Mouser lists 9 different 0.25 ohm 5W resistors in their on-line catalog for instance.And as far as replacements,10 ohm 1w,180 ohm 2w,450 ohm 5w and .25 ohm 5 watt..I may be able to get the 10 and 180,but .25 and 450,Ive never seen a .25, what would the equilivents be for those? ok I just checked them again before posting this, the .25 and the 10 ohm power are showing no real resistance,as well as 2 4.7 and 2 6.8 ohm 1/2 watt..maybe those are the culprits.But any advice, suggestions would be greatly appreciated by anyone who has operated on this species before.
There are some weak spots in the Thomas Vox amp designs. Of course, they were designed nearly 60 years ago, so it's easy for me to quibble with that now. If you've already replaced the power and driver transistors and electros, you have only the resistors to go and it's been re-manufactured entirely.
Check the power supply voltages on the power amp chassis if you haven't yet. In fact, a good diagnostic is to check and record the DC voltages on each pin of each transistor. I can probably spot most issues with the amp if you post those here. If you haven't done it yet, put a scope on it and look for oscillation on the driver collector. The use of a driver transformer means that the overal negative feedback can set these things singing, especially with new transistors in the pre-driver and driver position. Modern devices are MUCH faster, and this can be the nudge that makes it go into oscillation.
Check the DC voltage on the speaker output. With the driver transformer blocking DC from the preamp and driver, the DC output is completely determined by the values and matching of the resistors biasing the output transistors and the matching of the output devices themselves.
Anyway, there are some things to check. PM me if you want to dig into it in more detail than you can get posting here. Thomas Vox amps are something of a mania with me.Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.Comment
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Interesting enough, I am doing the same thing but with two Vox amps... A Royal Guardsman and a Viscount. I have been reading RG's excellent article on VOX amp repair at Geofex. Search for Thomas Vox SS Amp Tips. I can't wait to get these bad boys running.
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R.G. Keen of Geofex,bookmarked and visited frequently..brainfart ,for some reason I didnt connect the two..Technology of the wah set my gears in motion,thank you for that article...yes I am a lowly pedal tinkerer trying to play amp tech, and thus far the only success I ve had is because of you fine people..anyhow my work schedule bekons me so this is going to wait till the weekend,but I did what was suggested, lifted a leg on the resistors (the board is very brittle by the way) and surely R.G.has the roadmap of this terror..r302 is giving me 470 ohm is giving me 5-6 ohms at best,r309 6.8,is giving me 8 ohms,r311 is giving me 10, and both .25 wirewounds are zilch..so Im assuming,until I get these resistors,that this may be whats been causing chaos? And on the 1/8 watt resistors,would it be safe to play with 1/4 watt in their place? Because I have those out the yin yang. And I will check voltages this weekend as suggested..And cj,yes they are a bit of a project,especially for a novice like myself,good luck to you on your's..Well my job is calling and thanks to everyone for their suggestions and help.Hope you all have a great day/night!!Comment
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Your photos show two Viscount cabinets, one with a Royal Guardsman preamp installed. Is the Guardsman power amp also installed in that one as well?Originally posted by cjlectronics View PostComment
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I don't think that there are any 1/8 watt resistors or 1/4 watt resistors in any Thomas Organ Vox amp. Especially in the power amp section.Originally posted by dNi3L View PostComment
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oh my mistake,I meant to say 1/2 watt..would it be safe to swap those for 1/4 watt or should I play it safe? Dont want any more headaches with it than I have already..and thank you for replying 52 Bill.Comment
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Good question!!Originally posted by 52 Bill View Post
A client brought me these two amps and said, "if anything, make one good Viscount out of all the parts". I haven't had the time to research what I have. I thought the Guardsman cabinet is just like the Viscount cabinet. Does the Royal Guardsman have a different cabinet?Comment
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The Royal Guardsman I remember was a piggyback style with the speaker cabinet mounted in a trolly. I always thought of it as the Super Beatle's little brother.Comment
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Exactly, 60 watt power amp and a separate speaker cab with 2-12's and a Midax horn.Originally posted by Tom Phillips View PostComment
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There is some nice info about the Royal Guardsman and for the Viscount at the following sites:
The VOX Showroom - The Vox Royal Guardsman Amplifier
The VOX Showroom - The Vox ViscountComment
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You're correct. Both cabinets are Viscount cabinets. The model # on one is 1151 and the other is 1153 (this one has the speakers)
The 1151 is open back and the 1153 is closed back. The speakers do not look original.
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The Royal Guardsman's preamp connector fits perfectly into the power amp and it's power cord is clamped to the chassis. It almost looks like it is a direct replacement for the Viscount preamp??Comment
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Um....
Are we now running two different projects by two differentowners in the same thread?Education is what you're left with after you have forgotten what you have learned.Comment
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