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Hi - Lo switch - 725 VDC vs. 450 VDC?


Mike MacLeod
Posted on Friday, October 01, 2004 - 02:17 am:   

I see many references to the Hi - Lo switch when people are selling their 2275/2475 heads on Ebay or Harmony Central, and they invariably say that the high is 130 watts and the low is 65 - cutting out two of the tubes. I have not looked in back of my head to see if two of the firebottles are cut out of the loop, but I don't think they are, based on the schematics.

If you look at the two schematics on this site, you see that the switch is a coil tap for the power transformer. The high setting looks like it serves up the full 725 VDC to the 6CA7 tubes, but the Lo setting must produce less power. My theory is that the Hi setting is for the 6CA7s and the Lo setting is 450 VDC for EL34s. Perhaps that's why it sounds so good when the EL34s are being tormented on Hi.

The schematics also disagree about the tap line being wired to the hi or lo switch setting.

Can any readers shed more light on this? I apologize if this has been worked over in some other thread.

michael kaus
Posted on Friday, October 01, 2004 - 03:59 am:   

All the tubes still work but you are right, the voltage is dropped. It's not for the tube change, just supposed to be a volume drop. THe amps were designed with 6CA7's in mind. They later went to 6l6's because the supply of 6ca7's dwindled and they sucked too. Today, we have choices. JJ's are what I use and think highly of. Mike.
Steve Kennedy
Posted on Monday, October 04, 2004 - 03:14 pm:   

Yes, the High/Low power switch essentially cuts the output tube's plate voltage by almost half in order to cut the output power by half. In this way, Music Man could provide the same solution for the HD-series and the lower power series without any major design differences.

This has the interesting side-benefit of allowing "weaker" tubes (like most EL34s) to last longer in a Music Man amp and to allow the amp to run cooler. It does change the sonic signature of the amp somewhat but you never get something for nothing!

Another thing it allows you to do is to install the "Yellowjackets"-style of EL84 converters in a Music Man amp for a different tonal characteristic. These adapters (or the EL84 tubes) were not designed to operate at over 700Vdc, but they usually work fine at 400Vdc.


michael kaus
Posted on Monday, October 04, 2004 - 03:51 pm:   

I usually bias my MM 2-10 65 around 20-21 mA of idle current and this allows me to use the low power switch sometimes. Any lower and the amp goes into crossover distortion(too cold) and sounds like s%&t in the low power setting but fine in the high power setting. This is actually a really bare bones (and cool) way to bias your MM amp. Lower the bias down to the point it gets kind of ugly distortion on low but sounds good on high. Then bump it up a little until it sound ok on both. Real meatball but it DOES work. Not the way I would endorse all the time but in a pinch(in the field), it will do> Mike.
Steve Kennedy
Posted on Wednesday, October 06, 2004 - 12:15 pm:   

Works for me! I would say that the majority of the time I run my MM amps in the low power setting anyway! Of course, I am not currently playing out much so I rarely need full power.

20-21mA is towards the high side of the bias curve but shouldn't be greatly detrimental to the health of the output tubes. It should definitely sound better on Low Power!

Of course, this can vary between individual amps and different models. Be aware, be observant and be careful. If this is too high for your amp it will let you know in subtle ways (hot output tubes, tube plates dimly glowing red, etc.). Some of these symptoms may only be evident or visible in the dark!


Posted on Wednesday, December 15, 2004 - 07:29 am:   

I'm from the Netherlands: sorry for my English.
But, do you now if i can put yellow yackets in my HD onethirty head? I have to run the amp on the low power setting; i just read. But do you know if they fit?
John Drexler
Posted on Thursday, December 16, 2004 - 02:24 pm:   

Naw. Full power all the way, for me! Both my 65-112 and 65-210 suck at low power if you want a clean sound...and the tonality is DEFINITELY affected in low power mode. However, if you are going for a dirty, bluesy tone, then low power switch combined with the master volume on 8-10 gives a VERY true Chicago blues tone. Since I'm into clean, clean, clean, I don't mind replacing the tubes once or twice a year if needed...although I haven't noticed them getting ragged after more than a year of use in "high" power all the time. Again, a testament to the ruggedness of the MM amps...even on "high", they perform and hold up well. I agree, however, that proper biasing is essential, and also agree with Mike about the "cold plus a little" bias setting...use that technique myself. If the amp sounds good and the bottles ain't glowin' red and glass melting...that's about right for me!
Posted on Monday, December 20, 2004 - 04:31 pm:   

Nice story, but ik asked something about THD Yellow Yackets.
Posted on Monday, December 20, 2004 - 09:06 pm:   

I think John Drexler was responding to a different question, concerening the post prior to yours Stratokai...Steve Kennedy is one of the MusicMan experts and is the admin here... I think 425 volts on the low power setting would be the best bet for yellow jacket EL84 converters ...700 volts may fry them...700v just seems a bit higher than what most other amps and tubes run at...
Posted on Thursday, December 23, 2004 - 07:57 pm:   

Thank you Larry. You maybe right about Drexler. Sorry for the answer i gave. It's a forum. I'm not the only one.
With do they fit i ment the size. Is there enough space to put them in?
Posted on Friday, December 24, 2004 - 04:56 pm:   

I have the HD130 210 and they would fit fine...they also make a short version of the yellow jackets, but I have plenty of room for the Yellow Jackets... I dont own them, but I could about fir 10 inch long tubes in mine....because they point downward....
Posted on Saturday, December 25, 2004 - 04:32 pm:   

It's a bit easier to put em in a combo i guess!
I contacted THD (yellow yackets)
Normal size: 5inch
short version: 4 3/16 inch
In my HD130 head only the shorties will fit!
If its to off-topic someone tell me, but..'.
Larry do you think it's worth buying the yackets or not?

Posted on Sunday, December 26, 2004 - 09:17 am:   

I hear yellow jackets save on your power and output transformers, with less heat in your amp... if you like the Vox AC30 sound, then yes....they suppose to break up nicely and give you that classic British crunch sound(not for metal or punk sound), but since I dont own any nor ever tried one, then I cant say from experiance...many people rave about them and use 'em as well...also you can use 2 or 4 in a 4 tube output amp....what amp do you have anyway?
Posted on Sunday, December 26, 2004 - 09:21 am:   

ok, i see you have the HD130.... just try two and see if you like them, then get two more for the other 2 tube slots later...:=)
Mike MacLeod
Posted on Tuesday, January 04, 2005 - 05:52 pm:   

I got a set of yellowjackets for my HD130. It only takes about 5 minutes to swap them.

I only played them on the low (425V) setting. They have a lovely chimey sound, running Class A, and my Strat sounded quite lovely through them. They don't have anything like the oomph (bass) or the El34s, though.

The tone stack is also matched more closely to the EL34's characteristics. With the EL84s, there were some good settings, but many of the combinations I tried sounded more or less the same.

Unfortunately, I can't recommend the EL84s for overdrive sounds. They sounded OK with my Fulltone Soul Bender pedal, but I was unable to get much from playing with the preamp and master volume. Below a certain point they sound sweet and chimey; above that trebly and harsh. All in all, a specialized investment for studio situations, I think.

If I wanted a Matchless Lightning sound, I'd buy a clone kit from ceriatone.com.
Posted on Saturday, January 08, 2005 - 05:16 am:   

In stead of replace 2 EL-34 for 2 EL-84 (yellowyackets) i bought a small 15 watt tube amp with EL-84. The MM-130 stays original this way. And.. the way Mike Macleod describes the sound of EL-84 is exactly what i experienced. Nice for clean, but NOT for the overdriven sound
Posted on Saturday, January 08, 2005 - 11:09 am:   

15 watts?....Hope you get many months of service from it as smaller amps tend to BLOW up due to the small overheated componets...the wise thing would have been using heafty componets and getting many more years of amp life and installing yellow jackets... trust me, those small amps dont last!(im in the business and hear these stories)....Will