| Author | Message | ||
| bobkimball |
Insights from the experienced? 50W per tube is a lot for an EL34. Do 6ca7 handle 50W? I'm used to EL34 designs topping out around 30W per tube. I don't doubt Music Man's ratings, just amazed. I looked over the site and didn't find (I could be blind!) discussion on this. My recently acquired RP115 100 S/N DP00782 has JJ EL34s, tuned up by Bob Dixon, and it sounds FINE! | ||
| Steve Kennedy (admin) Username: admin Registered: 03-2002 |
I'll take a few wild stabs at it... Power is calculated in various ways (Ohm's Law), all depending on load impedance, current and voltage swing. In a traditional tube amp design (i.e. typical Marshall using EL34 tubes), your plate voltage is between 400-500vdc I believe. Music Man amplifiers use a higher than normal plate voltage (700-750Vdc, originally using the 6CA7 tube) so more voltage swing into the output transformer can generate more output power from the same tubes. It is not a direct translation of plate current and plate voltage because the tubes are driving a transformer winding (higher impedance), NOT the speaker load directly. The higher plate voltage implies a higher impedance primary winding which may be more efficient and stresses the output tubes less in terms of wasted thermal power as well. I'm not sure if I have the concept totally correct here (this is off the top of my head at the end of a long day with some creative fudging thrown in for good measure), but it seems that the higher plate voltages translates into lower plate currents and tube temperatures with higher ouput power in part due to increased transformer efficiencies (lower conversion losses). It isn't really 50 watts per tube, it is 100 watts for the pair in push pull through an output transformer into a speaker load. Traditional tube amps overload easier because of higher plate currents at lower plate voltages. The Music Man amps have much higher clean headroom built into the design so this all equates to efficiency improvements I think. Steve | ||
| bobkimball |
thanks Steve for taking the time to swing at it. I think you are in the zone. the way the output tubes are biased, they do run at low plate current compared to normal, and at a higher plate voltage. the DC power dissipation of the tubes is still in the normal zone for 6ca7/EL34. The sleight of hand, if you will, has to be in the AC power calculation taken at the output transformer, the tip off being the output power rating at 700V vs 450V supply voltage. It's not a big deal practically speaking because most people can't hear much difference between 2xEL34 or 4xEL34 in loudness. What is amazing is the successful use of the high voltage supply (with low voltage option) and the fact that the amp never sounds 'cold' even though the bias current on the output tubes is much less than in typical lower voltage designs. I'm curious why no one seems to have followed up on this design path. (well, there's always COST!)... thanks again for posting, Bob | ||
| Reed Ohrbom (reedmoh) Username: reedmoh Registered: 06-2006 |
Bob; If I've been correctly following the discussions about which MM amps came equipped with 6CA7/EL34's, (first generation), and which came stock with 6L6's,(1979/80 onwards) I thought that your amp would be designed for 6L6's. What tubes are listed on your tube chart? If your amp was designed for 6L6's, do you know if Bob Dixon had to do any modifications for it to use the EL34's? Do you know if there is much of a sound and/or power difference between using the 6L6's and EL34's? Thanks for your feedback. Reed | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
If you are considering the switch to EL34's, the first thing you have to do is take into account the additional heater current. The 6L6's pull about .9A (900mA) per tube whereas the EL's heater pulls about 1.5A (1500mA) per tube. That's a big difference when it comes to power drain on the power supply. MOST MM amps will take it but you will have to see if your transformer will take it. Other than that, I think you're ok. You will, of course, have to re-bias. I think EL's have pin 1 tied to someplace too, now that I think of it. You should pull up a sheet on EL34's and see if it's going to bother. Sometimes they use pin 1 for a tie point. (Note: Admin edited heater current numbers for accuracy.) |
Tue, 03/25/2014 - 09:29
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