Dual Rec and Marshall Questions Answered


Dual Recs & EL84s

What is the major difference between the Triple Rectifier 3-channel and the Dual Recto 3-channel?

Without getting overly technical the main differences are as follows: the Dual Rectifier uses four (4) 6L6 power tubes where as the Triple Rectifier uses six (6) 6L6s. As the model names indicate, the Dual Rectifier uses two (2) rectifier tubes and the Triple Rectifier uses three (3) rectifier tubes. The other notable difference would be the transformers. As we are often asked which model we prefer, I would recommend the Dual Rectifier over the Triple Rectifier and the main reason is the difference in transformers (especially the output transformers).


I was reading an article recently by the late/great Ken Fischer and he said that when it comes to Marshall amps with EL34s, it’s a good idea to change the stock 220K bias feed resistors to something lower, like 100K. This apparently helps prevent EL34s from wearing out prematurely. I had also read this same advice in Kevin O’Connors’ TUT1 book. I just bought myself a 1983 Marshall JCM800 2204 last week and had a look inside the chassis – it turns out that my amp does have these two 220K bias feed resistors. Should I replace those with a pair of 100K resistors the next time I go and retube the amp, or will something like JJ Tubes be fine with these stock 220K bias feed resistors?

Let me preface this by saying God bless Ken and may he rest in peace. Both gentlemen are technically correct in that it will lengthen the life of your power tubes. Should you change the 220K bias feed/splitter resistors to 100Ks? More than anything else, it comes down to personal taste (can’t stress that enough); while pages can be written on this topic, we do not have the necessary room with which to accommodate an in-depth technical explanation regarding how this impacts the phase inverter, the power supply, etc. So, suffice to say, we will only be scratching the surface.

Kevin’s books are fantastic and I highly recommend acquiring them, as they always contain a great deal of knowledge, required formulas, etc. and in my opinion they are well worth the investment. Authors presume that the information provided will be put to use through experimentation (i.e. trying various values, etc). In doing so, you will inevitably discover a wide variety of things that are not easily explained in books – in this case, changing bias splitter values – as a great many of these things directly impact the “feel” of an amp as well as the tone. Tone is subjective, therefore it is extremely difficult to describe the effects that various changes can have on tone as well as the feel of an amp.

Understanding how amps work (i.e. designing, modifying existing designs, repairing, etc.) is all about hands-on experience and nothing will ever take the place of spending countless hours on the bench experimenting. After all, it’s all about how the amp sounds and feels in the end, right?

Getting back to answering your question – yes, you can change the bias splitters to 100Ks, and in theory you can use any value between 82K to 220K, but of course it depends on the circuit design as whole. Does lowering the values from 220K affect the tone you presently have? Yes it does. Touch response/dynamics are altered, highend frequencies are attenuated (a high end roll off), headroom is effected, etc. One of the single most important things I can hope to impress upon you is that everything impacts tone.

Generally speaking, all JCM800s came from the factory with 220K bias splitters, as do nearly all amps that use the same basic push-pull power supply design as the JCM800 (far too many to list).

If it were me, I would leave the 220Ks in place, as I personally feel they sound and feel better than 100Ks. In addition, you may well have a JCM800 that runs on lower plate voltage and if so, my guess is if you changed them to 100Ks it would sound very muddy and undefined. But once again, tone is subjective so let your ears be the final determining factor. Good luck and have fun experimenting!


Have you ever converted Marshall (JCM, TSL, DSL) amps from EL34s to 6L6s?

Yes we have, and with regards to the JCM800s, some players prefer it. With regards to the TSL/DSL series, I do not think that it’s worth the while. What I would recommend is going with a larger bottle EL34 such as the EL34B/Ruby EL34BSTR (same tube); it has more bottom end than the typical EL34 and has the open top end much like a 6L6.




Trace Davis
President/Founder of Voodoo Amplification Inc.
www.voodooamps.com
www.myspace.com/voodooamps
45 Atwater Road, Lansing NY 14882
Tel: (607) 256-0465 Fax: (607) 330-0272
Email Trace at: trace@voodooamps.com

Tube talk and turkey

That wonderful day we all know as Thanksgiving has once again come and gone. I trust that it was a relaxing time for you and that you were able to spend some quality time with family as well as close friends. I’d like to take a moment to share a little something with you that will hopefully enrich your life outside of the world of tube amps; this year, for the first time, we purchased a free-range turkey for Thanksgiving dinner. A free-range turkey is a bit more costly but much like plugging into an amazing boutique amp or playing a well-crafted guitar; the difference was truly a wonderful experience. Give it a try and you will be glad you did.


Hello, I work on my own amps and can change caps, bias tubes, etc. and I was wondering what your thoughts were on how different coupling capacitors effect the tone and what you prefer to use? Thanks! DocZ – Denver, CO

Hello Doc,
Unfortunately the differences in capacitors (caps) and how they impact the tone goes well beyond the scope of this article and could easily constitute a book on the topic. With that said: we stock a great deal of caps for wide variety of reasons. The caps we would use for a vintage Marshall restoration would be different from the caps we would use for a vintage Fender or Vox restoration.What I am getting at is that all caps do sound and feel different from one another.

The more simplistic the circuit design, the more of a difference you will hear with caps. By comparison, high gain amps tend to cloud the differences a bit, but that’s not to suggest that the differences cannot be heard. The quality of the transformers is something that is almost always overlooked in cap discussions, as it is the last thing most techs truly understand. For example; take a Marshall reissue plexi (1959x/1987x ) and experiment with various coupling caps on V1 only. Now replace the output transformer (OT) with a Mercury Magnetics plexi spec OT and run the same experiment again. You will find that the differences are more pronounced with the higher quality Mercury OT. You will also find that the caps you favored the most with the stock OT may not be the caps you will favor with the Mercury OT.

We are also often asked if the bypass caps make an audible difference (IE: caps in parallel with the cathode resistors in the preamp section) and the answer is a resounding “yes,” as not only do they effect the tone but they also effect how the amp feels and/or responds. Do the results differ from using a polarized / electrolytic (e-cap) bypass cap verses a non-polarized cap? Yes it does.We’ve also experimented with countless e-caps and the differences can be night and day.We stock SBE Orange Drops (715P, 716P, PS), Aricaps, 150M Series (formerly Mallory) in the polyester / polypropylene, Solen, Hovland MusiCaps, some Xicon, etc. Bottom line, there is nothing like hands on experience to order some caps and dig in!


My friends and I have been bitten with the tube bug.We’ve been trying all kinds of NOS as well as modern production tubes in our amps and while we’ve found some tubes we like, they are often times too noisy. My friend has a 2-Ch Rectifier (Mesa), another has an original block letter 5150 and I have a Peavey 6505 plus. Do you have any suggestions or is this just the way it is?

NOS tubes are made differently than modern production tubes and the reasons for this are too numerous to list without getting overly techie. Suffice to say that modern safety standards prohibit tube manufactures from making tubes the way they once did, and there are steps being taken to improve on modern production tubes. Many will tell you that the V1/the first tube in the signal chain is the most critical, however in my opinion all the tubes are critical to tone. If you want to reduce the noise in a high gain amp you can experiment with various tubes for V1, but be mindful of the tonal differences as well as the level of noise that you are trying to reduce. High gain amps will have a certain amount of running noise/hiss to them that is often times associated with how the gain is structured within the gain stages, etc. (i.e. a 4-gain stage high gain amp will generally not be as quiet as a vintage 2-gain stage vintage amp will be). Try using a Groove Tube GT-5751-M in the first stage and see where that takes you. Often times this reduces the noise floor while still keeping the punchy bottom end and harmonic detail most look for from those amps.

Have a great month!


Trace Davis is the president of Voodoo Amplification Inc., which is considered to be the world’s leading modification company and one of the foremost experts on vintage Marshall amplifiers. Trace is also an experienced session player, touring player, recording engineer and producer.

Trace Davis – Voodoo Amps – www.voodooamps.com
Email Trace at: trace@voodooamps.com
45 Atwater Rd, Lansing NY 14822
(607)256-0465

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Copyright (c) 2006 Trace Davis
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