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Post by countrysinger on Oct 14, 2014 8:31:51 GMT -7
I recently purchased this used z28 head from my local music store and just found out it's running 5881 tubes instead of the 6v6's. It sounds great, but can I switch to 6v6's or was it modded to run the 5881's?
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Post by Maddog on Oct 14, 2014 10:25:02 GMT -7
Yikes!!! You better get your voltages checked before switching back to 6V6's. There's an old trick used with old 6V6 Fender Deluxe Reverb amps where you "sub-in" 6L6's, a bigger rectifier, a few resistors and caps, and crank the plate voltage. I would make darn sure this amp has NOT been modded before you go back to stock (6V6) power tubes....
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Post by Mark (Basement Enthusiast) on Oct 14, 2014 13:39:34 GMT -7
^ Lee makes a very good point.
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Post by countrysinger on Oct 14, 2014 15:51:08 GMT -7
I guess I better take this little bugger back to the store tomorrow and get them to check it out.
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Post by simpleton on Oct 14, 2014 16:25:05 GMT -7
keep us posted. What a bummer to not just get to go merrily on your way.
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Post by smolder on Oct 14, 2014 18:06:47 GMT -7
If you're handy, pull the chassis and either post a photo of the circuit or send it to the Z boys so that you know if it's been modified. If not, then you can just have 6V6's installed and the bias set. Of course, a good tech could do all that as well.
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Post by countrysinger on Oct 15, 2014 11:58:14 GMT -7
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Post by smolder on Oct 15, 2014 12:25:52 GMT -7
It's hard to see, but the resistor that is in series with the small trim pot (this adjusts the bias) should be 33k... Three orange rings. I'd forgotten that the doc puts 50k trim pots in (fender's are typically 10k) which should be plenty to span the typical difference in bias between 6V6 and 5881 (low 20's to mid 30's mA). I doubt that there was a mod for that change.
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Post by countrysinger on Oct 20, 2014 12:47:50 GMT -7
I took it back to the music store and they swapped the tubes to jj 6v6's and set the bias. Now I find the amp super bright. Even on my neck pickup with the treble all the way down on the amp. Is this normal? I loved the sound of it with the 5881's and now regret getting them to put the proper tubes in.
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Post by "Z" Steve on Oct 20, 2014 13:00:15 GMT -7
The JJ's are more like a 6L6 and are not my choice of tubes for the power section. But, it could be possible that a modification still could have been done. How about the other tubes - original or replaced?
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Post by countrysinger on Oct 20, 2014 13:10:21 GMT -7
The rest are original as far as they told me. I thought jj's were what these amps came with?
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Post by Christopher on Oct 20, 2014 13:31:38 GMT -7
They may have come w JJs stock but a lot of us either run old glass or other branded 6V6s that are more 6V6 in nature than the JJ version which is akin to a baby 6L6 ergo more top end. I have a NOS pair of GE 6V6s in mine. The Tung Sol reissues aren't bad for modern tubes either. Sorry that there's too much top end. My ASAT Classic loves the '28 and it's a great compliment to my LP. Hope you find a happy medium that works for you.
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Post by smolder on Oct 20, 2014 14:30:12 GMT -7
I'm also not a fan of the JJ's, they are really designed more like 6L6's... that said, the only reason I can think of them adding brightness is if the 5881's were worn out. Seems like power tubes wouldn't effect the tonal range much, but when they wear out, I'm told the top end goes first. If you are in Colorado you could come over, try some 6V6's, and compare your's to my Z28.
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Post by John on Oct 21, 2014 10:16:14 GMT -7
I took it back to the music store and they swapped the tubes to jj 6v6's and set the bias. Now I find the amp super bright. Even on my neck pickup with the treble all the way down on the amp. Is this normal? No that doesn't sound normal. I had a Z-28 for a while. Compared to the Doc's other amps, it is not known as a bright amp. I don't know of anyone who runs it with the treble all the down. This doesn't smell right. Again, keep us posted. Where in general are you located?
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Post by countrysinger on Oct 22, 2014 11:48:02 GMT -7
I'm in Calgary, Alberta.
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Post by John on Oct 23, 2014 5:34:46 GMT -7
That's not near anyone I know on the forum. Perhaps if someone IS near that area with a Z-28, they'll speak up.
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Post by johns62312 on Oct 26, 2014 19:57:09 GMT -7
Why does it have mismatched screen resistors? One looks like a half watt.
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Post by Maddog on Oct 26, 2014 20:15:47 GMT -7
Hmmmmm. That def does not sound right, However, I'm not well versed on the Z-28 guts....This one might be worth sending back in to the mothership for a "once-over."
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Post by countrysinger on Oct 26, 2014 20:32:52 GMT -7
It has mismatched screen resistors?
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Post by Albert on Oct 26, 2014 23:17:45 GMT -7
It has mismatched screen resistors? Pm me an email address I'll take mine apart and send some detailed pics of the guts ...
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Post by Albert on Oct 27, 2014 10:09:47 GMT -7
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Post by countrysinger on Oct 27, 2014 13:00:37 GMT -7
Well, isn't that something? Why would mine be different? and would the difference resister cause?
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Post by headshrinker (Marc) on Oct 27, 2014 13:41:17 GMT -7
It's possible a power tube went and took out the resister (which is common) and then someone replaced it. As long as it's the same value it doesn't matter if it looks different. Look at the color bands on your blue one and see if the ones on the tan one in the same location on the other tube match. If they do you are good to go. If not it needs replacing. Might check the other resister on the tube also but from what I can see in your picture it looks like it matches. I can't see the color bands on the other well enough to tell. I would also check your color bands against the ones in Albert's pics.
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Post by johns62312 on Oct 27, 2014 14:55:23 GMT -7
I think I would much rather they be the same wattage rating, there is probably a reason they are 2 or 3 watt. Anybody know what the wattage on these are.
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Post by Albert on Oct 27, 2014 20:16:27 GMT -7
I think I would much rather they be the same wattage rating, there is probably a reason they are 2 or 3 watt. Anybody know what the wattage on these are. 1/2 watt 2.7k resistor and 2 watt 470 ohm resistor
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Post by "Z" Steve on Oct 27, 2014 20:41:48 GMT -7
I don't know what I'm looking at or what for, but THAT is a fabulous looking piece of art. How much more meticulous could the production of an amp be folks? Frame it, give it gallery lighting, and a name.
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