Random Thoughts

Here I have assembled a variety of ‘random thoughts’ (hence the name of this ‘Lesson’), that did not fit neatly into any of the other ‘Lessons’, nor was there enough to warrant an entire ‘Lesson’ of its’ own. Some of these thoughts may not weigh high on the ‘ponderosity’ scale, whilst other ramblings may lead to an epiphany or two. These are just idle musings, and it is up to you to further investigate and decide on the merit (if any) to these ‘random thoughts’. Ready? If you have your thinking caps on……………

  • Tube testers command the big bucks on eBay, yet the humble VTVM fetches pennies in comparison. A tube tester is nothing more than an overpriced Ouija Board, yet anyone who works on tube equipment without a good VTVM is not being the most competent technician. Why isn’t the VTVM being discussed as a valuable (if not the most valuable) piece of test equipment? I have added a ‘Lesson’ on the VTVM, which can be found by CLICKING HERE. Other ‘nice to have’ test equipment includes a capacitance meter, an inductance meter, a capacitor leakage tester, and an ESR tester. The other piece of test gear you should have is the impedance meter I show you over at CLASSIC ARTICLES. If you want to check out just the impedance meter construction article, CLICK HERE.
  • ‘Ground’ switches on your vintage amplifier are really ‘Death’ switches, yet few gurus advocate their complete removal. Even fewer would have you check the capacitor for leakage. Perhaps they do not own a VTVM or a leakage tester? In today’s world, a .1uF ‘line bypass’ capacitor would not meet with CSA or UL approval. In any vintage Fender amplifier I service, the ‘Ground’ switch becomes ‘inactive’. Period. Ask your local guru if he knows what the CSA or UL limits are for having AC current on the chassis. A ‘hot’ chassis was acceptable in the AC/DC days of Radio and TV, but times have definitely changed. Another thought; how many ‘gurus’ advocate changing over to a 3-prong ‘grounded’ AC plug, yet perform the procedure ‘wrong’, and neglect to mention about disabling the ‘Ground’ switch? For the absolutely worst way to do this, check out the video ‘Basic Tube Guitar Amplifier Servicing and Overhaul’. It’s basic all right; basically wrong. For a detailed description on the absolute ‘best’ way to do this procedure (and really, the only way to do it properly), CLICK HERE.
  • Thankfully, using ‘cloth-covered’ wire does not get the reverence it once did. Some manufacturers ‘reissue’ the cloth-covering, and make a fortune selling what is called ‘vintage’ wire. Unfortunately, cloth-covered wire is useful purely for aesthetics. And, just like other reissues, they ain’t got it ‘right’. ‘Original’ cloth-covered wire actually had a polyethylene inner insulation; modern ‘reissue’ types use Teflon. I don’t want to even start thinking about tonal difference between ‘solid’ versus ‘stranded’ wire. But my original question still remains; ‘Why was cloth-covered wire ever talked about at all’? Many people talk about speaker ‘phasing’, and how it effects tone. Phrases have been coined, such as having a ‘forward playing’ amplifier. Yet every Reverb equipped Fender has each channel out of phase compared with the other, as do many Bassman heads.
  • On a closed-back cabinet (e.g. Marshall), do people really think they can hear the difference between a plywood back and a particle-board back? Would a Pine back board sound even better?
  • Speaking of Marshall, doesn’t anyone else see the sheer stupidity in fusing the bias supply? (This only applies to Canadian imported amplifiers!) This almost makes the guru-approved practice of fusing the center-tap of the output transformer seem like a stroke of genius by comparison. The stark reality is that both practices are only recommended by very uneducated people pretending they know what they are doing with tube amplifiers. Many of my friends are amazed at the amount of catalogs I receive.
  • They come from all sorts of businesses, and in a very roundabout way, they all help me build and repair guitars and amplifiers. Small hand-tool catalogs are essential. Remember this; no tool is specifically designed for guitar makers. Keep your eyes, ears, and mind open, and you’ll reap the rewards. Think about who other than you might need such a ‘widget’, and where that person might shop. A couple of quick hints; try catalogs aimed at scale-model builders and find at least one catalog that sells automotive fasteners. I get many comments on how over the years my amplifier chassis has ‘featured’ unusual fasteners. ‘Cage Nuts’, ‘Crimp Nuts’, and ‘Rivnuts’ are only unusual if you’ve live a sheltered life.
  • Always ‘double check’ any information you acquire through books or websites. Yes, this includes the very website you are ‘reading’ right now. You should also think about your own skill level before tackling a project, or asking questions that rate in the ‘Beginners Oughta Know’ category. Technical errors do happen, and of course there is a lot of misinformation out there. BBS ‘chat’ areas are notorious for having flat-out technical inaccuracies, and folks asking some very disturbing questions. I still seriously think about adding a ‘Lesson’ of misinformation culled from books and the internet. Not that my website is perfect, although I do try and correct any mistakes I make. Also, some people ask questions that make you wonder. Here are a few sample ‘Question and Answer‘ queries. The first I pulled from a popular website for vintage radio collectors.

Q: What is the difference between Transconductance and Mutual Conductance tube testers? 

A: Transconductance is a gain test where signal output from a tube is measured. Mutualconductance, like used in the Eico 666 tube tester , is a type of emission test. Instead of all elements being tied together, like most emission testers, they use different voltages on elements.


Hopefully you can appreciate the point here. No one will lose an eye because of the little white lie told here, but my attitude is that if someone gets a part of simple tube theory wrong, how can you trust any of their other answers? I know diddly about transformer theory, but I know enough to keep my mouth shut past a certain point. Any advice you aren’t 100% sure about should always be preceded with the caution…. “I am not sure, but I think…..”. This saves a lot of embarrassment later on. Next up, this question actually appeared on a very popular BBS chat area.

Q: What does a Selenium Rectifier do for a ’59 Fender Super amplifier? What can replace it? I am looking into making one of these amps, but this part is not around anywhere.


 Ouch! It seems everyone who knows which end of a soldering iron to hold is tackling their own tube amplifier project, and that’s where the trouble begins. The next example eavesdrops on a poor fellow that owns a Bassman head (who obviously would benefit from reading the ‘Lesson’ on Bassman heads).

Q: I have a Silverface Bassman, I believe it’s the AB165 circuit, but I haven’t completely verified that yet. Anyway, I have a 100-ohm cement block resistor coming off of both of the cathodes of the power tubes to ground. No Bassman schematics I’ve seen have this. I’m sure it’s a mod of some sort, but I can’t figure out it’s purpose. Also, I can’t bias my tubes past about 29-30ma (I think). I have the pot turned all the way up. Now none of this is a problem for me, since I like the tone. Am I correct in assuming the reason I can’t bias it hotter is because of the 100 ohm resistor? And why would someone put a resistor that high in a fixed bias circuit?
A: I believe that should be a direct ground. I’ve put 1 ohm/2w resistors in there and done a voltage drop across them to determine bias. Don’t know why there are 100 ohm resistors in that one. I’m sure one of our resident experts will answer this.

A: You have a ’68 circuit in that amp. I would suggest that the circuit be returned to ’64 specs in the bias and power section. BF amps take the cathode to ground without the resistor. BF with the ’64 schematic amps also have a true bias section as opposed to the balancing circuit that the ’65 and ’68 schematics have.


Hopefully, you see the first fellow shouldn’t be prying inside his Bassman. The second person is only slightly more astute. The third person in on the conversation finally has it ‘right’, although no one yet has solved it completely. Another fellow who shouldn’t be poking his nose in a tube guitar amplifier laments his woes to another popular BBS chat area below.

Q: I think one of the Screen resistors of my JCM800 combo has destroyed itself. Anyway, if one of the screen resistors has blown itself away (my multimeter says its around 30K) how would that affect the amps sound?
A: If a Screen Grid resistor is broken, then the Screen Grid won’t get any voltage and the output for that tube will be about zero.

Q: I whipped out the Screen resistors, and just went ‘straight’ with a pair of clip leads, and voila. A quick test yielded (comparative) tonal Nirvana, though I haven’t tested it too much since I didn’t want to blow up my tubes. Anyhow, I examined the thing for a while and noticed that before I did this, one tube seem to be doing very little and the other was…well… kinda blue a lot.

A: The Marshalls that ran without Screen Grid resistors had a reputation as the best sounding amp in the world – for about 15 min.! The Screen Grids, especially in modern tubes, sometimes cannot handle the current at full output. I would install fresh Screen Grid resistors before doing any more testing. 



p style=”text-align: left;”>The fellow answering seems knowledgeable, but strangely holds a lot of information back, including advising the would-be technician to let a ‘professional’ deal with his Marshall. Lastly, here is the epitome of what can happen when too much (mis)information falls into the wrong hands.


Q: The Tremolo ‘Intensity’ potentiometer in a friend’s BFVR starts working only from ‘6’; in other words the Tremolo does not work at all at any setting between ‘1’ and ‘5’. The ‘Intensity’ pot is stock. Furthermore it worked just right until a few months ago. The optoisolator is brand new. The V5 tube is okay. I have checked all the connections in the Tremolo circuit, and everything seems OK. Even the pot seems okay; it shows the expected readings with a DMM. All the cathode bypass capacitors and the plate load resistors were replaced a few months ago with Atom Spragues and metal films.
A: Give us a reading of V5’s cathode voltages, and you might also check the plate load resistors for drift.

Q: These are the readings on V5 with the Tremolo switched ‘off’: Plates: 371VDC on pin 1, 308VDC on pin 6; Cathodes: 0VDC on pin 3, 5.9mVDC on pin 8. The grid on pin 2 reads -51VDC. These readings are far from the schematic value.

Do I really need to comment?

Please; make sure of your skills before taking on a project. If you can, begin your education with simple circuits, and doing simple maintenance work. You can progress to simple modifications on ‘tag boards’ as opposed to the printed circuit boards seen in many Marshall amplifiers. During all of this, supplement your tube theory and circuit knowledge before you assume a higher level of circuit manipulation. You can only benefit in the long run.

That’s about it for now. As I come up with other ‘Things that make me go hmmmmm…..‘ I will definitely add to this list.

You can email me any other ‘Random Thoughts‘ of your own, and I’ll be sure to include them as well.

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