Post by myles on Sept 23, 2008 13:58:56 GMT -7
This is a question I received over at Guitar Player and I thought it might be of interest to some here ....
Hi Myles!
Newbie alert! I have recently purchased a Rivera K-Tre 55-watt combo purely on the basis of the spectacular clean channel. When I got the amp home, I discovered it actually had a dirty channel too -- one for which I hold no particular love. I've tried a number of things with greater or lesser success:
- I returned the combo to a near-original tube setup (12AX7WBs across the board), and the clean was great, but the dirt was staccato as opposed to smooth
-I replaced the tubes with 12AX7EHs across the board, and the whole thing went dark
-I put back the 12AX7WBs and added a 12AX7EH in position 1. It all got better, but I still find it a bit staccato-sounding
For what it's worth, my other rig is driven by a JMP-1 in which the dirty preamp is run through a 12AX7LPS. It is by far (to my ears) the smoothest gain/overdrive I've heard so far, and is more or less the quality I'm going for in the drive side of the Rivera. Alas, the LPS does not do particularly well in combos.
I'm about to order another raft of tubes (probably just in pairs for position 1 and 2) and I'm looking for advice. I'm considering Tung-Sols, JJs, and in an interesting twist, 5751s.
Anything else I might like to try?
Joe
Joe,
The biggest problem in swapping preamp tubes is one does not know what an off the shelf preamp tube is doing. Trying to compare a 12AX7EH to a 12AX7WB will yield untrue results if the WB is close to design spec and the EH is not close. You need to compare tubes that have their specs close .... transconductance, true gain, plate resistance and current output.
You said that the EHs made the amp dark and this tells me you had some weak ones as the EH is brighter and more articulate than a WB.
A recent test of a lot of EH's showed this:
1.0 1160 0.014 82.9
1.1 1430 0.016 89.4
1.1 1510 0.017 88.8
1.3 1610 0.018 89.4
0.8 1260 0.014 90.0
0.8 1120 0.013 86.2
1.3 1580 0.0165 95.8
1.3 1560 0.0165 94.5
1.1 1540 0.017 90.6
1.1 1540 0.017 90.6
0.8 1310 0.015 87.3
1.2 1570 0.0165 95.2
1.3 1540 0.017 90.6
0.9 1370 0.015 91.3
1.1 1470 0.017 86.5
1.2 1590 0.017 93.5
0.6 1160 0.014 82.9
0.8 1280 0.014 91.4
1.0 1430 0.0155 92.3
1.4 1670 0.018 92.8
0.8 1200 0.014 85.7
0.9 1290 0.014 92.1
1.1 1480 0.0165 89.7
1.2 1490 0.016 93.1
0.8 1340 0.015 89.3
1.0 1540 0.0165 93.3
1.0 1370 0.015 91.3
1.3 1620 0.018 90.0
0.8 1300 0.0145 89.7
0.6 1170 0.013 90.0
1.0 1500 0.017 88.2
0.9 1370 0.0155 88.4
Output TC Gain
1.0 1418 90.09
84.9% 88.6% 90.1%
1610 100.6%
1120 70.0%
30.6%
1.4 116.7%
0.6 50.0%
66.7%
95.8 95.8%
82.9 82.9%
12.9%
This might not make a lot of sense but the important factor here is that these vary 66.7% in regard to spec current and the average tube is down 15% from spec and that is like turning a 50 watt amp into a 30 watter depending on position in the circuit.
Transconductance has a 30% spread and this is not good either.
You need to be able to compare apples to apples as today's production preamp tubes are a crap shoot and the odds are NOT in your favor.
Due to the inconsistent nature of tubes 65amps has me blueprint every amp that goes out. Here is an example ... I am sure the formatting will get hosed up but maybe you will get the idea:
Monterey
S/N M2H 809089
Current TC Gain Ref number
1.7 1720 0.004 43.0 917-17 Dario EF86
1.1 1610 0.0175 92 917-26 JJ803S
1.2 1770 0.019 93.2
1.1 1320 0.015 88 919-18 Tung Sol 12AX7
1.1 1510 0.016 94.4
36.1 JJ 6V6S
37.1 JJ 6V6S
Hi Myles!
Newbie alert! I have recently purchased a Rivera K-Tre 55-watt combo purely on the basis of the spectacular clean channel. When I got the amp home, I discovered it actually had a dirty channel too -- one for which I hold no particular love. I've tried a number of things with greater or lesser success:
- I returned the combo to a near-original tube setup (12AX7WBs across the board), and the clean was great, but the dirt was staccato as opposed to smooth
-I replaced the tubes with 12AX7EHs across the board, and the whole thing went dark
-I put back the 12AX7WBs and added a 12AX7EH in position 1. It all got better, but I still find it a bit staccato-sounding
For what it's worth, my other rig is driven by a JMP-1 in which the dirty preamp is run through a 12AX7LPS. It is by far (to my ears) the smoothest gain/overdrive I've heard so far, and is more or less the quality I'm going for in the drive side of the Rivera. Alas, the LPS does not do particularly well in combos.
I'm about to order another raft of tubes (probably just in pairs for position 1 and 2) and I'm looking for advice. I'm considering Tung-Sols, JJs, and in an interesting twist, 5751s.
Anything else I might like to try?
Joe
Joe,
The biggest problem in swapping preamp tubes is one does not know what an off the shelf preamp tube is doing. Trying to compare a 12AX7EH to a 12AX7WB will yield untrue results if the WB is close to design spec and the EH is not close. You need to compare tubes that have their specs close .... transconductance, true gain, plate resistance and current output.
You said that the EHs made the amp dark and this tells me you had some weak ones as the EH is brighter and more articulate than a WB.
A recent test of a lot of EH's showed this:
1.0 1160 0.014 82.9
1.1 1430 0.016 89.4
1.1 1510 0.017 88.8
1.3 1610 0.018 89.4
0.8 1260 0.014 90.0
0.8 1120 0.013 86.2
1.3 1580 0.0165 95.8
1.3 1560 0.0165 94.5
1.1 1540 0.017 90.6
1.1 1540 0.017 90.6
0.8 1310 0.015 87.3
1.2 1570 0.0165 95.2
1.3 1540 0.017 90.6
0.9 1370 0.015 91.3
1.1 1470 0.017 86.5
1.2 1590 0.017 93.5
0.6 1160 0.014 82.9
0.8 1280 0.014 91.4
1.0 1430 0.0155 92.3
1.4 1670 0.018 92.8
0.8 1200 0.014 85.7
0.9 1290 0.014 92.1
1.1 1480 0.0165 89.7
1.2 1490 0.016 93.1
0.8 1340 0.015 89.3
1.0 1540 0.0165 93.3
1.0 1370 0.015 91.3
1.3 1620 0.018 90.0
0.8 1300 0.0145 89.7
0.6 1170 0.013 90.0
1.0 1500 0.017 88.2
0.9 1370 0.0155 88.4
Output TC Gain
1.0 1418 90.09
84.9% 88.6% 90.1%
1610 100.6%
1120 70.0%
30.6%
1.4 116.7%
0.6 50.0%
66.7%
95.8 95.8%
82.9 82.9%
12.9%
This might not make a lot of sense but the important factor here is that these vary 66.7% in regard to spec current and the average tube is down 15% from spec and that is like turning a 50 watt amp into a 30 watter depending on position in the circuit.
Transconductance has a 30% spread and this is not good either.
You need to be able to compare apples to apples as today's production preamp tubes are a crap shoot and the odds are NOT in your favor.
Due to the inconsistent nature of tubes 65amps has me blueprint every amp that goes out. Here is an example ... I am sure the formatting will get hosed up but maybe you will get the idea:
Monterey
S/N M2H 809089
Current TC Gain Ref number
1.7 1720 0.004 43.0 917-17 Dario EF86
1.1 1610 0.0175 92 917-26 JJ803S
1.2 1770 0.019 93.2
1.1 1320 0.015 88 919-18 Tung Sol 12AX7
1.1 1510 0.016 94.4
36.1 JJ 6V6S
37.1 JJ 6V6S