Hey All,
Was just thinking if anyone has built an amp using these tubes in the power section. If so, what can you tell me about their tone, application (PP, SE, bias setup, etc.). These tubes look interesting, but I have zero experience with them.
Donations
Anyone built a 6973 or 7868 power section?
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dartanion
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novemberrain
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Yeah. Whaddaya wanna know?
The 7868s are 7591s with a different base. Use 7591s, octal sockets are nicer and you'll never have the 7868 problem of the little pins breaking or chipping the glass on the tube base.
Applications: Look at early '60s Ampeg schematics, you'll see 7591s in the earlier amps and 7868s in the later ones, Reverberockets, Jets and such. Cathode bias 'em. Look up Hammond "E" or "H" series amps to see other 7591 applications. Fisher used the 7591s in some of their stereos. Bogen used 7868s in some of their stuff.
6973s... well, I avoid 'em because they're semi-rare. Got an old Supro with 'em, plus a couple Kinsman organ amps. They're tougher than EL84s, higher ratings all around. Same 9 pin mini socket, different pinout. If ya got a pair build sumthin' wif 'em!
The 7868s are 7591s with a different base. Use 7591s, octal sockets are nicer and you'll never have the 7868 problem of the little pins breaking or chipping the glass on the tube base.
Applications: Look at early '60s Ampeg schematics, you'll see 7591s in the earlier amps and 7868s in the later ones, Reverberockets, Jets and such. Cathode bias 'em. Look up Hammond "E" or "H" series amps to see other 7591 applications. Fisher used the 7591s in some of their stereos. Bogen used 7868s in some of their stuff.
6973s... well, I avoid 'em because they're semi-rare. Got an old Supro with 'em, plus a couple Kinsman organ amps. They're tougher than EL84s, higher ratings all around. Same 9 pin mini socket, different pinout. If ya got a pair build sumthin' wif 'em!
Last edited by novemberrain on Mon 12/19/05 6:36 pm, edited 1 time in total.
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novemberrain
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Yeah, to my ear 7868s sound like 7591s. I really like the 7591s, they're a relatively high powered tube in a package the size of a 6V6. There's also an example schematic in the back of the RCA RC30 tube manual for a 35 watt phonograph amp, come to think of it there's a 7868 amp in there, too.
7591s are right in between 6V6s and 6L6s, you get the 6L6 clean with a little less of the 6L6 volume. IMHO within their ratings 6V6s are a bit dirtier (than 7868s, 7591s or 6L6s) when pushed...
IMHO the 6973s are cleaner than EL84s, more along the lines of 6CM6s or 7189s.
Measured plate voltage in my Supro (using 6973s) is over 400 volts, IMHO that's a hostile environment for EL84s!
The Supro is a lot dirtier than a Marshall 18 BUT it's an entirely different amp (than the Marshall), grid leak bias on the peamp and all. Lotsa cheap ceramic caps in there. Topography-wise the Supro uses one stage of a 12AX7 for a grid-leak biased preamp stage, a cathodyne phase splitter (another 12AX7 stage), then the dual 6973 output stage. I'd have to look again to see what the cathode bias resistor is...
I'd have to dig the thing out of storage, I haven't seen it in quite a while!
Come to think of it, Leslie used 6973s in the reverb amps for 122Rs, 145Rs, and 147Rs. It's still surprising that they didn't use 7189s. I'd have to take another look at a Leslie 251 amp, I think there's 7189s in there for the treble channel...
7591s are right in between 6V6s and 6L6s, you get the 6L6 clean with a little less of the 6L6 volume. IMHO within their ratings 6V6s are a bit dirtier (than 7868s, 7591s or 6L6s) when pushed...
IMHO the 6973s are cleaner than EL84s, more along the lines of 6CM6s or 7189s.
Measured plate voltage in my Supro (using 6973s) is over 400 volts, IMHO that's a hostile environment for EL84s!
The Supro is a lot dirtier than a Marshall 18 BUT it's an entirely different amp (than the Marshall), grid leak bias on the peamp and all. Lotsa cheap ceramic caps in there. Topography-wise the Supro uses one stage of a 12AX7 for a grid-leak biased preamp stage, a cathodyne phase splitter (another 12AX7 stage), then the dual 6973 output stage. I'd have to look again to see what the cathode bias resistor is...
I'd have to dig the thing out of storage, I haven't seen it in quite a while!
Come to think of it, Leslie used 6973s in the reverb amps for 122Rs, 145Rs, and 147Rs. It's still surprising that they didn't use 7189s. I'd have to take another look at a Leslie 251 amp, I think there's 7189s in there for the treble channel...
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novemberrain
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Correction: I just looked at a Leslie 251 amp, that uses 6973s (not 7189s) for the "treble" channel. 400 volts plate voltage, shared 300 ohm cathode resistor bypassed with a 25/25 cap, 18 volts on the cathodes. I couldn't tell you what Leslie used for a plate to plate impedance, I can tell you that the little grey Triad transformers they used in that application don't live up to Triad's reputation for reliability...
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dartanion
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What model Supro is it that you have? Didn't Jimmy Page play Supro amps for a while? Did he play a 6973 equipped Supro?
TDLS has a few Supro schematics, however one that I was looking at was labeled Gretsch on the schematic. I don't know much about these off brands, so any help would be great. Are there any killer off brand amps that might be a cool build (besides a WEM Dominator of course)?
TDLS has a few Supro schematics, however one that I was looking at was labeled Gretsch on the schematic. I don't know much about these off brands, so any help would be great. Are there any killer off brand amps that might be a cool build (besides a WEM Dominator of course)?
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RWood
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I've got a set of each in my Ampegs - a '64 Gemini IV that uses a pair of 7591A's and a '66 SB-12 that uses two 7868s.So 7868s will sound just like a 7591 correct?
For lead work, I much prefer the tone of the 7591A's, especially when cranked. Nice overdrive. They have a reputation for their 'clean hifi sound' and as Novemberrain said they were used heavily by Scott, etc, in their stereo amps, but those tubes get nice 'n nasty in a guitar amp turned to 11.
The 7868s, even in a tube rectified circuit with the same preamp design, have a nice sound at low volume but are too 'spanky' for me at high volume. Not as smooth as the 7591As.
Just my 2c
RWood
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