Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on August 09, 2013, 11:31:31 am
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At this point I have the amp as a 8/4 ohm combo with the 16 ohm wire (yellow) terminated. The EXT speaker jack , not knowing the term name, has built in blades to switch to the 4 ohm wire (brown) when the plug is in the jack. I'd like to know if I can separate the 2 jacks. One for 8 ohm only and the same goes for the EXT jack to be 4 ohm only. I'd have to still run the ground to each jack
Thanks
http://www.freeinfosociety.com/electronics/schematics/audio/sunn2000s.pdf (http://www.freeinfosociety.com/electronics/schematics/audio/sunn2000s.pdf)
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Yes, you can do that. I would replace that special switching jack with a non switching jack.
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Just to be sure, then there's no need for a toggle and I can have individual jacks and I will mark them
But the one ground wire still supplies both
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That's correct. But you only use one jack at a time. IOW, don't connect one cab to the 4Ω jack and also connect another cab to the 8Ω jack at the same time.
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Perfect, that's what I want
Thanks!!!
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When using separate jacks for the 4 and 8 ohm taps, don't use shorting/switching jacks for EITHER jack.
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yes, one wire doth not a circuit make.....I think William Fender Shakesandrumbles said it 1st, or was it Marvin Gaye, & Tammy Terrell.......It Takes Twooooooooo,............. It Takes Twooooooooooo green, and blue....or whatever colors. They don't have to rhyme, but they do have to complete the circuit. I'm really sorry for this indulgence, just trying for some humor, and using the mnemonic device I used to remember. definitely no sarcasm intended. looking at schematic, it appears that terminating 16ohm tap also terminates nfb, but I 100% differ to the waaaay more qualified forum members. (I'm a newbie, albeit old in age myself)
And thanks for the pic, it brought back memories of my Uncle w/Elvis. He had that setup, up in the top attic portion of He,and my Aunt Vivian's house, up in Salt Spring. Should have seen me, as a kid of about 15-16, getting that cabinet down the tiny narrow staircase by myself.
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sorry , didn't know you had been gotten back to. didn't want you to be sittin there, w/open cab, wondering what to do. Sluckey, we haven't been 'introduced', but hat's off to ya. you don't fool around.
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Just for reference, here's a Sunn schematic showing the funky switching jack...
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I installed to 2 new jacks and then when taking out the amp a saw clouded and cracked broken tube. Plus I noticed a 47 ohm (2 watt) resistor was blown on the tube closest to the output transformer .
I replaced the resistor and installed another tube in the broken one's spot. To bias, I use Weber bias right which is a great item to use. Checking the Voltage side (Vp) all 4 tubes are getting just under 500 volts. Moving the switch to Ik, only the tube next to the output transformer and the tube 2 spots away sent out a reading. The other 2 are sending a negative reading. Typically all 4 tubes will have a reading and then I can tweak the bias pots. The way this amp runs is the outer tubes are tied together and the inner tubes are tied together but I always get a reading from each tube
Can anyone tell what my issue is? Plugging in the bass there is sound and the amp is super quiet, noise free
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Forgot to mention that the tube closest to the output transformer is running so hot that I had to shut the amp down. Also 2- 47ohm - (5w) resisitors are gone too. One from each pair
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Sounds like you definitely have a problem there Rich. Could be a bad socket...one that has arced, but there are other things to check before assuming that. First thing to do is make sure the negative voltage for the bias is getting to the grid of each power tube. It should be the same on each tube. If you don't have bias at the tube, then backtrack the supply to see where it stops...could be a bad resistor or cap in the bias supply. Also check to make sure the tube sockets are properly adjusted for each tube so each tube makes contact on the pins. Please also make sure there is no voltage on the pins before you re-tension them. Something is making those 47 ohm resistors blow.....and if you did have a bias problem and the tube in that socket had run hot like you said, then that tube is probably now suspect....and possibly others in the quad too. Check B+ on pin 3, ground on pin 8, the proper screen voltage on pin 4, and negative bias voltage on pin 5. The absence of a voltage where you should have one, or perhaps a voltage lower than what it should be could indicate what is going on. A leaky cap between the phase inverter and power tubes could cause the negative bias voltage on pin 5 to be too high (close to zero as compared to whatever negative number it should be) and would throw the bias off. Have you changed the filter caps or signal caps in this amp at any point?
Greg
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HI Greg
You're right , looking again I saw a filter cap that was all bubbled around the edge and body. Before I saw that I tightened up the sockets. I have a Sencor tester so I tested to see if any of the tubes were bad and they were fine. The tester only checks leakage , emissions & shorts. At this point because the list is growing that I may bring it a local guy I know
As Always
Thanks
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I looked at the layout again and I checked the bias resistors for both pots. . Thanks to the forum I have 2 bias pots , one per pair. The resistors were fine. Not knowing how the bias caps were and being low $$ I bought a new piar and installed them . Plus I installed 2 new- 47 Ohm- 5 w resistors. Also, the cap that I thought was gone, wasn't
Well, I got lucky because it turned on with not even a whisper. I think that cracked tube was item that did the damage. Then I whipped out my trusty Bias Rite and got to work. I have probably 6- GE 6550A's and only 2 of them would be considered a match. So I biased those up and moved on to the next pair. From my balance of GE's I couldn't get a pair that was close so I tried 2 others. I had pair of new Tung Sol 6550's and a tube I never heard of but it's out of a VHT guitar amp and marked VHT KT-88. I haven't used KT-88's for while so I set up both and then listened to the amp with the GE/Tung Sol combo vs. the GE/KT-88 combo. Honestly I liked the the GE/KT-88 combo better. At lower and at louder ranges. I know it may be a no no to mix but it sounded good and both pairs are biased in the middle of the range they could be set for
So I'd like to thanks all you guys for the help!!
PS- has anyone tried the new Tungsol KT-120??? It;s supposed to be great. Let me know and is it compatible with the Sunn 2000s?
Thanks
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Glad you appear to have got it sorted Rich. Running 6550 and KT88 together isn't a problem as long as each tube biases up in a good range.
Greg
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Greg
Any feelings on the KT-120? Is it a drop in for the 2000s?
Thanks
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Greg
Any feelings on the KT-120? Is it a drop in for the 2000s?
Thanks
I haven't tried it Rich. No idea.
Greg