bmusic - Tech Tips

Valve Amp Q and A
by Bob Charman

Q.
Hi Bob,

I'm JC in North Queensland.
I have a Fender Super 210 - 60 watt amp 1990 -1992 which I can't seem to find anyone here to fix correctly and am after some advice on. The amp blurs on some double notes without being very loud, it's not the speakers and if I turn it up a bit it's worse. I have used my friends 60 watt Fender valve amp and it is great.

This amp has not been knocked around and is in near new looking cond.
The technician says it sounds OK and the valves and bias are right but obviously this is not so. I am thinking of putting new 6L6 power valves in to see if this might fix it.

Could you give me some advice please?
Regards,
JC.

A.
JC,

Without seeing and especially hearing the amp, there is a limit to the advice I can give you. BUT, there are a few things I'd check before putting out the dollars for a pair of 6L6's.

1. Check that it isn't the speakers, I know you said it isn't, but this is the most likely cause of what you describe. This can easily be done by connecting another speaker box in place of your existing speakers, or even by swapping the speaker connections with your friends amp.

2. Check that it is not dirty contacts in the effects loop, especially on the power amp in connector. You could check this by making up a shorting plug. You do this with a stereo jack plug and connect the tip and the ring connections together, NOT the sleeve or earth connection. Plug this into the power amp in socket and see/hear if the sound improves. If it does then the power amp in connector needs cleaning or replacing. This is a more common fault than you would believe, especially when the effects loop is not use frequently.

3. Try substituting a known good 12AX7 into the preamp, do one valve at a time, (this way you only need to buy or borrow one 12AX7) to see if this improves the sound. IMO this is more likely the cause than the 6L6 power amp valves. When you say you tried your friends amp, is it a Super 210 or some other Fender model. If it is a different model then it is likely that it will sound different. The Super 210 has a fairly high gain preamp section which will tend to give a 'dirtier' sound.

Finally, if you do change the 6L6's remember to rebias the amp, or if you are not sure how to do that get an amp tech, who is familiar with valve amps, to do it for you.

Good luck with it.

Rock and Roll is the last refuge of the politically incorrect.

Q.
Hi Bob,

I was hoping you could help me. I'm soon to be the proud owner of a Victoria amp with a US power trannie and I would greatly appreciate some advice on step-down trannies. I've seen your previous response regarding these and called Wagner electronics. The trannies they stock are Toroidal which is good, less hum, run cooler etc. My concerns though are threefold, they are made in China, they are earthed on the input side but not on the output and they are 115v. The Chinese manufacture I would think is not an issue as they are made to an Australian standard (apparently) and of course that makes them cheaper, the earthing is a concern as the amp has a ground terminal, does it matter that it's not connected to anything and therefore not grounded? Does the output voltage of 115 volts matter as the amp spec reckons it needs 120 volts?
Regards,
GB.
A.
G'day,

I've used a number of these without any problems.
Made in China, what isn't these days? As you say, they have to meet Aus Standards so there shouldn't be any issues.
115 vs 120 is not enough to be significant. The USA used to be on 110 volts but, I believe in the 70's, they changed to 120 volts, so a lot of manufacturers went for the halfway point and marked their kit 115 volts, which is closer to reality anyway when you allow for the average voltage drop in US houses.
Since they use a lower voltage than us, the current is higher to give the same power. Voltage drop is a factor of current flow, so they suffer higher voltage drops on their mains than we do. BTW in Oz a voltage drop of 12 volts in our 240 volt mains is considered acceptable.
The reason that it is not earthed on both sides is to maintain absolute separation between the two sides, this actually makes it safer than if both sides were earthed.
Good luck with the Victoria, I've heard good reports on them, very much after the style of the Black Faced Fender amps, I believe.

Rock and Roll is the last refuge of the politically incorrect.

Q.
Part 1
Hello Bob,
Got a quick q for you. I have just bought (second Hand) and in as new condition, a Laney210 30 w vs30 Amp. I am a ludite when it comes to valve amps. I can see that the valves are a sovtec brand. I play Shadows type music. I find I am not quite getting a nice warm sound even when the bass knob is turned to the full positon. I turn the trebledown to around 3 but I feel that the sound is too "tinnie" Is there vales that I can get to give me that warm sound or does the Amp just need a good service (bias) etc.
Your thoughts would be appreciated.
Regards,
Tom

A.
G'Day Tom,
It is really hard to give you a very meaningful answer without actually hearing the amp. There are a variety of things that could be causing the problem.
It could be the valves, Sovteks do tend to be a bit bright. You may want to try some JJ's (which are available through bMusic), they tend to be closer to the 'older' style sound. The preamp valves will have the most effect on the tone, so try swapping these first, they will be 12AX7 (ECC83). A lot of modern guitarists like a lot of treble boost, so valve manufacturers tend to lean that way.
It could also be the speakers. Laney is a UK manufacturer and generally uses Celestions or similar, which have a fairly bright sound.
Or it could be that the tone circuits in that amp are not what you really want, in which case you will need to have a tech change the components in the tone circuits. This is not a simple job and you really need to have a tech who has experience in valve amps to do it properly.
Finally, it could be the guitar. A lot of recent guitars have overwound pickups which give a treble heavy tone, for the same reasons as above.
Sorry I can't be more helpful than that, tone is something that you just have to hear to get a handle on.
Regards,

Q.
Part 2
Ive just had a look at the 2 preamp valves in the Laney and they are Ruby E83CC.
I don't know whether they are good or just ordinary.
Regards,
Tom

A.
Tom,
Ruby's are rebadged valves obtained from the cheapest supplier at the time of a production run. If it were me I'd invest in a set of Svetlana or JJ, 12AX7's. I would just about guarantee that your amp will sound better.
Contact bMusic for a price on either of the above.
Regards,

Q.
Hi there Bob,

I’ve just purchased a Silver Face Fender Vibro Champ from the USA and wondering the best way to convert it to Australian voltage standards? Will a step down transformer work or should I get it modified internally? Can you please tell me my options, and what is the best option if its no hassle. If it involves changing any of the vintage parts inside my amp I’d rather avoid it, but if its just a change of capacitors and resistors???
Any help would be greatly appreciated.
Regards,
Steve.

A.
G'day Steve,

The easiest, and in my opinion the best way is to use a step down transformer. The reasons for this is that a) it is cheaper and easier, b) you are maintaining the original vintage value, c) no modifications internally (BTW a real vintage purist would consider changing the resistors and caps as degrading the vintage value, and changing the power transformer to 240 VAC would be a definite no, no). You can get a suitable step down transformer from various suppliers, I personally use WES (Wagner Electronic Services) in Sydney. Make sure that you get one that will easily handle the power drawn by your amp, this information will be on the back plate, usually near the power cord. If you are going to transport the amp to gigs I would suggest that you mount the step down transformer inside the amp cabinet, keep it as far away from the speaker magnet as possible. Then you can leave the power cord plugged into the step down and just use the cord from the step down as your mains cord.
Have fun with the SF VC, a very nice amp, IMHO.

Q.
Hi Bob,

Thanks for the info on a step down converter for my vibro champ. I also have another question in regard to a Fender Super Twin 180w valve amp. It’s in very good condition cosmetically but its distorting almost straight away when I put the volume past 4, and the the master on only 2. It’s got some hum and is NOWHERE near as loud as it should be. I know the 6 x 6L6’s need replacing. Is that why its got bugger all power anymore? This thing should be ear bleedingly loud and clean on these settings. The valves are pretty far gone, they’ve got burn marks on them, and 2 of them don’t light up as much as the other 4.. is this the only reason its losing so much power? It does get loud, but a Fender Hotrod with both volume and master on 2 is louder than this thing with both settings on 4. any help would be much appreciated.
Thanks once again,
Steve.

A.
G'day Steve,

Like you say, the ST should peel the paint off the walls and certainly shouldn't be distorting until it is really cranked. If the 6L6's are old they could be part of the problem but I would also check the power supply capacitors and the voltages around the output valves. There are a number of other things that could cause the problems, possibly pre amp valves, interstage capacitors, dry solder joints and without being able to have a look it is hard to say which it might be. If you replace the output valves, and get them properly biased, and the problems still exist it would be worth getting a tech to have a look at it. However, be very sure that whoever you take it to is familiar with valve amps, a lot of techs are not, so ask around and find one who does this sort of work.

Q.
G'day Bob,

I am going to change the tubes in my Engl Savage 120. And i want to know can I use different brand types of preamp tubes together.It takes 6 but i want to use a different tube for the first stage. A Mullard in the first and Sovteks in the rest?
Also would i have to use the same brand of tube for both power amp and preamp stages?
Thanks,
Alan

A.
G'day Alan,

You can mix and match as you please with pre-amp valves. It is important though that you use a matched pair or quad in your power amp valves, but they can be any brand that you prefer, they don't have to be the same as the pre-amps. Remember, if you change your power amp valves you may need to have them re-biased, depending on the circuitry used in your particular amp.

Q.
Hi Bob,

Hi,
My 900 Marshall 100watt has started to make what sounds like a fart when the bass hits with distortion. and also the clean channel isn't as clean as it used to be. It this pre-amp valves or power valves? Also it there any valves that you would suggest for this amp?
Thanks,
G.

A.
G'day George,

It could be either pre or power amp valves, though I would suspect power amp would be more likely. It could also be the power supply capacitors need changing. My personal preference is for Winged C Svetlana or JJ valves, but it really is a personal choice thing.

Q.
Hi Bob,

I am just about to replace the valves in my Peavey 5150 head and wondered what valves to use, I am only replacing the power amp tubes at this time and need advice on what tubes to use. I love Zakk Wylde's amp sound, i know he uses 6550's and wondered if I could put these in my 5150. Would that work?
Matt

A.
G'Day Matt,

It could feasibly be done but it would require rewiring the entire output section and changing all the biasing components and possibly the output transformer. Not really practical.

From the land of big crocs, hard rocks and dirty socks keep rocking.

Q.
Bob,

Many thanks for your Q&A page.
I bought three little Kent brand valve amps (1960's I think) on ebay for a song. I took them to a tech here in Sydney (Pete Reynolds) and he said:
"The Kents have no power transformer. They derive the high voltage from rectifying the mains voltage. This is "extremely" dodgy/dangerous - but a very economical way to punch amps out. In plain terms it means if you plug your guitar into the amp and turn it on - when you touch the strings you have a fist full of lightning (the mains voltage). In the case of Yank amps that means around 120Vac."
He's recommending installation of an isolating transformer in each which is pretty expensive. Is there any other way I can use these safely?
I want to set up a stereo chorus rig for home recording with a pedal in front of two amps.
Ronnie

A.
Thanks for your question Ronnie,

"He's recommending installation of an isolating transformer in each which is pretty expensive. Is there any other way I can use these safely?" NO!
I wouldn't even power them up without an isolation transformer, and they probably have a two pin, no earth, plug which I would change for a three pin before I started.
Not only that but you will need a 240V to 120V stepdown transformer anyway, and you can get those in isolating format, however, not all step downs are isolating so be careful.
If you really want to use them, get an isolating, step down transformer fitted and tested by a good tech like Pete Reynolds there in your area.

From the land of big crocs, hard rocks and dirty socks keep rocking.

Q.
Gday Bob,

Do you know what would happen to an amp if the guitar had a small dc offset along with the ac output signal to the amp? lets say a dc offset of 0.5 to 2.0 volts dc (most extreme case).
By my calculations it should really only underdrive the amp and it would be insanely loud because the ac signal would still be there but instead of the sine wave being +0.3 volts to -0.3 volts peak to peak (about) the sine wave would be +1.3 volts to +0.7 volts peak to peak. Would that excess dc voltage actually damage the amp? or would I just have to be really careful not to dammage the amp?

Thanks

John.

A.
G'Day John,

What would happen is that the DC offset would act to change the bias on the first pre amp valve, I'm assuming we're talking valve amps. Since in most amps this is a 12AX7 or similar in which the bias is only 1 - 2 volts a DC offset of the size you are talking cut either cut off or saturate the valve depending on the polarity. The AC signal would not be greater, since it is just sitting on the DC. If you have this problem due to a pedal or something you could either, fix the pedal, or put a DC blocking capacitor in the output of the pedal.
Hope that helps.

From the land of big crocs, hard rocks and dirty socks keep rocking.

Q.
Gday Bob,

Hoping you have the time to answer a quick question. Seem to be getting electric leakage, which my amp is picking up (peavey encore, just serviced, new valves and speaker). Works fine in the shop, but getting an annoying hum at home.

Is there any products out there which can filter out some of the hiss prior to it getting to the amp?

Thanks

John.

A.
G'Day John,

There are so many different thing that could be the problem.

For a start are you in Oz?

Many countries in the world, including the USA, have now or have had in the past, two wire mains circuitry which was a common cause of problems like this, as well as the electrocution of a number of guitar players. A lot of pre 1980 amps from the USA, (Tweed, Blonde, Brown and Black face Fenders all have this), have two wire mains circuits unless they have been modified.

Is it really 50/60 Hertz hum or 100/120 Hertz transformer buzz?

It could be induced from fluorescent lights, including the new compact fluorescent lamps (CFL's), or low voltage halogen down lights.

It could be bad earthing in the house, or the MEN not properly connected at the fuse/circuit breaker panel.

It could be a faulty guitar lead or the wiring in the guitar.

It could be picked up from plugging the amp into a socket that shares a circuit with a fridge or freezer.

It could be faulty power supply caps.

It could be a microphonic valve that only plays up when it gets hot.

Really, I could go on, but without 'being there' it's almost impossible to say with any certainty. Since the amp works at the shop, it is most likely an earthing problem at home, but who knows?

Sorry, that's not much help.

From the land of big crocs, hard rocks and dirty socks keep rocking.

Q.
Hey,

Have you ever added pots to a peavey ultra 112 to adjust the bias with the control grid? I am working on adding some pots to the amp to match the output of the tubes and when i get them matched, a third pot to change the bias to get rid of crossover distortion.

Thanks,
Jim

A.
G'day Jim,

With all due respect, if you have to ask questions about this sort of work then you shouldn't be doing it. This is a very dubious mod. I would first want to know what you expected to gain from this mod, it probably won't result in a significant difference in the sound of the amp, over what can be achieved with the current circuit. It will obviously void any warranty and has the potential to cause serious harm to the amp and the person doing the job.

If you are really serious about doing this I would need to get access to a copy of the circuit before I could comment further, but my advice would be DON'T DO IT.
From the land of big crocs, hard rocks and dirty socks keep rocking.

Q.
G'day Bob,

Thanks for making such interesting and useful information freely available.
Respect!

I have a Peavey Ultra Tube 112 (which I quite like) that is described as "self biasing" in the manual. Can this be true?

Cheers,

Keith

A.
G'Day Keith,

Several Peavey valve amps, as well as many others, use a technique called 'Cathode Biasing', sometimes referred to a 'Self Biasing'. Since valves are "normally on" devices, a trick can be used to bias them without having to supply a negative DC voltage source to the grid. If a resistor is placed between the cathode and ground, and the grid of the valve is referenced to ground (usually by connecting a large value resistor, such as a 1 Meg, from grid to ground), the valve will try to conduct a large current from cathode to plate, since the grid and cathode are initially at ground potential. However, this cathode current flow will cause a voltage drop across the cathode resistor, making the cathode voltage positive with respect to the grid. Since the cathode voltage is now positive with respect to the grid, the current flow will decrease, and the valve will head back towards cutoff. A point of equilibrium will quickly be reached where the increase in current is offset exactly by the increase in cathode voltage, and the bias current will stabilize at some particular value. It will remain at this value unless the resistor value is changed, or a different valve with different characteristics is plugged in. This allows the desired bias point to be set by varying the value of the cathode resistor. (Largely taken from Aitken's Reference Pages - a tube amp resource of general knowledge but widely quoted and recognised as having material best translated for the layperson).

So, you might ask, why aren't all amps Cathode Biased?

Well, cathode biasing is not without its faults. It turns out that in order to keep the DC bias supply voltage at the cathode constant while the input signal is changing, the cathode resistor must be bypassed with a large capacitor. This capacitor effectively "shorts" the AC signal component to ground, while allowing the DC voltage to remain relatively constant. If the capacitor is removed, the cathode DC voltage will have a signal voltage superimposed on it, which will subtract from the grid-to-cathode signal voltage, and reduce the gain of the stage.

The problem comes in when there are large signal level changes, and the average DC level of the cathode voltage changes. This causes a bias shift, usually in the direction of a colder bias point. This bias shift can be audible, but is sometimes desirable for guitar amp use, as it adds varying harmonic overtones to the sound. If the bias shift is severe, the valve will go into cutoff, and large amounts of "crossover" distortion will occur. In addition, the current flow through the cathode resistor generates a necessarily large bias voltage on the cathode for proper tube operation (typically 30-50 volts for most higher power output tubes). This voltage subtracts from the total plate voltage, which decreases the available output power. Between this voltage decrease and the bias shift, the output power in cathode biased operation is reduced when compared to fixed bias operation. As an example in a fixed bias amp an EL34 usually has about 400 volts DC between the plate and the cathode with the grid biased at minus 42 volts with respect to the cathode. In a cathode biased amp the 400 volts is shared between the plate and the grid, so that there is 42 volts beween the grid and the cathode and only 358 volts from the cathode to the plate. Since it is the plate to cathode voltage that influences the power amplification there will be significant reduction in power compared to a fixed bias amp with the same valve and voltages. Therefore, fixed bias is usually used for higher power amplifiers (50W and higher), and cathode bias is usually used for lower power amplifiers.

From the land of big crocs, hard rocks and dirty socks keep rocking.

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