service & mods to the Peavey ‘Valve King’ 100 amplifier

Peavey Valve King 100

Peavey Valve King 100

Hello. This week we received at the workshop one of the more recent Peavey made-in-China valve amp heads for repair. One of the ‘Valve King’ models, this is the VK100. Stated fault was – the amp just stopped. We have been repairing Peavey amps since the 1980’s, by the way, including warranty work, but we have noticed a steady decline in the serviceability of their amps and will only accept non-warranty repairs these days.

Valve King 100 head

Valve King 100 head

The amp indeed would not power up, and examination of the mains fuse, actually the only external fuse, revealed that it had not blown, so the next step was remove the chassis from its sleeve. The fuse for the HT (high voltage) supply, F201, is located on the main p.c.b. Obviously one or more of the 6L6GC power output valves had gone S/C (short-circuit), causing the HT fuse to blow.

Valve King 100 head

Valve King 100 head

Unfortunately, F201 is a miniature glass fuse directly soldered into the p.c.b. (printed circuit board), so easy replacement is out of the question – the board has to be stripped out, ie removing all control knobs, nuts, screws, etc. Is this for real ?? As this fuse is likely to blow several times during the life of this amp, the first MOD we will implement is to wire the fuse to an external fuseholder on the chassis rear panel, easily accessible to all. The HT fuse is T1.6A. You can see this MOD completed in the photos, plus the Dymo labelling.

Valve King circuit boards

Valve King circuit boards

One of Peavey’s favourite design tricks is to wire the valve (tube) heater filaments in series rather than parallel, even including the output valves. There can be various combinations of series heater arrangements in the one amp. Without a detailed schematic diagram on hand, this can be very confusing, especially in amps with a higher number of valves, eg the 6505 combo. Basically, if you remove any valve from its socket, the other valves (or possibly just some of them) will not power up. For each Peavey model, we build up a file of such idiosyncrasies so that we don’t get caught out twice.

HT fuse MOD has been implemented

HT fuse MOD has been implemented

So, we have reached the point when we intend to install a matched quad of JJ 6L6GC output valves. These valves should sound very good in this style of amp. The next logical step after installing any output valves is to rebias the amp. However, the bias supply in this amp is not adjustable, nor is there any provision to monitor the current draw of the 6L6’s. HUH ?? The idea that anyone would design a 100 watt valve amp with a non-adjustable bias supply in this day & age is beyond comprehension, but that is the situation with the VK100.

miniature pcb fuses & bias resistor MOD

miniature pcb fuses & bias resistor MOD

The amp initially appeared to be slightly under-biased, so we replaced R211 with a different value which resulted in a correctly biased quad of JJ’s. Next time this amp comes in, we will install a 25-turn trimpot in place of R205, which will solve the problem once and for all. We also installed 470K resistors across high voltage supply capacitors C202 & C204, as these caps were holding their charge (nearly 500V DC) long after the amp was switched off, complicating the service job. This has no effect at all from the customer’s point of view, but makes life easier for the next service tech.

VK100 rear panel

VK100 rear panel

The remaining tasks are an electrical safety test (PAT test), power output test and burn-in test, with a final play test to complete the job. The amp delivered 30V into an 8 ohm resistive load, which translates to 112 watts. When we first attempted a power output test, we couldn’t get a good clean sine wave output, even from injecting our test signal at the FX Loop ‘return’ jack. This was most perplexing, until we discovered the mysterious Texture pot on the rear panel, near the speaker jacks.

the mysterious 'Texture' control

the mysterious ‘Texture’ control

The Texture control claims to sweep the power amp from Class-A (c.c.w.) to Class-A/B (c.w.) mode of operation. This is complete nonsense ! What it actually does is convert the amp from normal push-pull (Class A/B) operation to a kind of quasi single-ended mode by shunting away some of the signal from one of the two outputs of the phase-splitter stage to ground. This only roughly approximates the behaviour of a single ended amp and naturally the resulting waveform is somewhat distorted, which is what caused our initial confusion. The only way to carry out a meaningful sine wave test is to set the Texture control to fully clockwise (c.w.). Well, how does the amp sound with a decent set of valves ? Not too bad actually – the ‘clean’ channel sounds very Fendery, with a slightly different EQ sweep, and the ‘lead gain’ channel actually starts off with considerable distortion levels, so any kind of ‘crunch’ tones are just about impossible, but will probably please the younger guys, and the reverb is also pretty good for a short tank driven by IC’s rather than valves. We also provide world-class service to the Peavey EVH amps and the well known Classic 30/Classic 50 models. Thanks for checking in to the blog – I.R.

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5 Responses to “service & mods to the Peavey ‘Valve King’ 100 amplifier”

  1. Noel Jaasma Says:

    Hi,

    I agree totally that the servicability of a lot of (cheap) modern amps is really bad. I just got the same vk100 in for repair…OT has blown (13 ohm/19 ohm dc) and no ac 10 khz balance on the anode windings when i put a signal in the speaker jack. To my experience this OT trouble happens too often with peavey amps, i don’t know why this is.
    I allready had to replace a 220 k PI anode resistor a couple of months ago. Have to check if there is a relation between these defects. The texture knob is indeed a waste of space…

    Problem is that this is a practise room amp and it is used by many bands.

    Regards

    Noel

  2. Dustin J. Albright Says:

    I have this amp. Although I am not a professional amp repair man I do have a fair amount of electronics repair experience and have repaired quite a few solid state and hybrid amps in the past.My amp is blowing fuse f 203 and diodes 206-208. I am just not familiar with tube circuitry. I am quite comfortable with capacitor discharge and all the high voltage stuff and the filter caps held quite a charge for a month without use. I made a discharge tool with a resistor and a led bulb as a visual Que. Anyways to the point any idea what to investigate in my case. I believe from my research it is the low 14v heater circuit . I was just going to revalve it and replace the diodes and fuse but I am currently disabled and on just my wife’s income so with the price of tubes these days I done want to just start throwing parts at it. Any leads would be awesome. Thanks in advance.
    Dustin J. Albright
    Woodwerker1995@yahoo.com

    • ivanrichards Says:

      sorry Dustin, we can’t undertake to talk readers through individual specific faults, but have published your comment for other readers of the blog; as a matter of policy we don’t encourage readers to work on high-voltage valve (electron tube) circuitry unless they are trained or otherwise qualified to do so; IR.

  3. Dustin J. Albright Says:

    Thank you for your reply. I am certainly qualified to work on this type of circuitry and however I understand you cannot confirm my experience and have to,well to put it bluntly cover your ass. Like I mentioned I have repaired numerous solid state units but have most of my experience in televisions,stereos and computer circuitry. Mostly tv’s. I have however got to the root of my problem which is valve five arced fried R1 and took out the low voltage heater circuit Fuse 203 and diodes D206-D209. Strange finding however resistor R1 calls for a 2watt 5% 330ohm. There was 2watt 1% 100ohm in its place. I am assuming it was something to do with bias on one hand but on the other hand suspect it may have caused it’s failure possibly allowing the tube to run beyond the rated wattage but I am guessing on that one. R1,R2,R5 &R6 are screen resistors and are all supposed to be 330 ohm. Could be a factory oops or I really suspect I was sold a reconditioned amp as new. It had mismatched preamp tubes in it as well. The amp always had a smell to it. I thought the smell was new amp smell when I first got it but I never thought much of it cause it wasn’t really the burning board smell it was more of a warm vinyl smell. Anyways starting to ramble I hope someone can use my troubleshooting info just waiting on new tubes to slap it back together. Most likely going to do the bias pot mod on it and going to go with the recommended 330 ohm screen resistor for R1 unless I uncover a reason not to in my bias measurements. Does anyone know the best bias measurement method for this amp. Not a whole lot of info on the subject since they are a fixed bias amp. I have a biasing manual for Carvin amps I imaging that should do. The transformer shunt method seems to make the most sense to me seeing as you physically check each individual tube. Ok I’m out way to long of a post. Thanks in advance for any info people.

    Dustin J Albright

    • ivanrichards Says:

      hello Dustin; have published your comment for our readers; you are completely correct – I DO have to cover my back-side in today’s litigation-prone environment ! IR.

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