Im thrilled with what I am getting from the Herron. It is a pleasure to work with Keith Herron as I explore the capabilities of the VTPH-2. I'll share what Keith has told me, but dont get the idea that this preamp is fussy. Keith is just thoroughgoing with his advice. Ive done nothing more than place four new 12AX7EH tubes at V1 through V4 to get most satisfying results.
There are two versions of the VTPH-2
(2 x 12AX7, 3 x 12AT7) at 64 dB gain or (4 x 12AX7, 1 x 12AT7) at 69 dB gain, MC input
Quote
A customer sent one of our VTPH-1 MC Plus phono stages in for repair (a broken RCA input connector, the unit was 12 years old) last week and they had a set of four 12AX7EH (Electro-Harmonix) tubes in the unit. I had not heard this tube before and it was quite impressive in terms of sound and low noise (much quieter than the current tube set in the VTPH-2). I have since recommended these to a few people and the word is that is quite and improvement. I had a brief listen with them and I will have to get some in myself. :-)
I did try the 12AX7EHs in a unit like yours (2AX, 3AT) and the sound was very good. (I left the 2 additional capacitors in place)
The extra gain with four 12AX7EHs may or may not be a problem for you with a 30K Ohm load. The only way to know is to give it a try.
I would not bother with getting a matched set, just buy a few extras and make and check to see that the channels are balanced after you install them.
If you need to you can switch them around to match the channels. Also you can adjust the gain and bias for channel matching. Setting the bias counter clockwise increases the harmonics and decreases the gain, going clockwise does the opposite. We usually set these around the middle (12:00 o'clock).
The twin triodes in the 12AX7s and 12AT7s are used in reverse order for left and right in the VTPH-2 so measuring the tube gain could give a different result for a given tube in opposite channels.
Leave the EI 12AT7 (V5) at the output.
Thanks for the note. The smaller plates on the 12AX7EH keep the grid-to-plate capacitance at a lower level than it would be with conventional larger plate 12AX7s (such as the EI and Telefunken tubes). The RIAA ends up being very close with the 12AX7EH tubes on both the four 12AX7 and two 12AX7 units (the 12AX7EH is in the middle capacitance wise). The 12AT7EHs in your unit have the larger plates.
The total effective grid-to-plate capacitance is a product of the gain (mu) of the tube and the actual grid-to-plate capacitance. In this case your 12AT7EHs have less gain (mu = 60), but more plate size than the 12AX7EHs (mu = 100). The effective capacitance is close enough (less than 0.2dB error at 20kHz and this can be corrected with the RIAA trim pots if you wish)
End quote
The tubes I am using are:
original EI 12AT7 at V5
Electro-Harmonix 12AX7 Gold Pin from Tube Depot
Testing : Low Noise and Microphonics
I've compared this preamp to the ARC PH7 extensively and prefer the Herron. The ARC is almost twice the price.
There are two versions of the VTPH-2
(2 x 12AX7, 3 x 12AT7) at 64 dB gain or (4 x 12AX7, 1 x 12AT7) at 69 dB gain, MC input
Quote
A customer sent one of our VTPH-1 MC Plus phono stages in for repair (a broken RCA input connector, the unit was 12 years old) last week and they had a set of four 12AX7EH (Electro-Harmonix) tubes in the unit. I had not heard this tube before and it was quite impressive in terms of sound and low noise (much quieter than the current tube set in the VTPH-2). I have since recommended these to a few people and the word is that is quite and improvement. I had a brief listen with them and I will have to get some in myself. :-)
I did try the 12AX7EHs in a unit like yours (2AX, 3AT) and the sound was very good. (I left the 2 additional capacitors in place)
The extra gain with four 12AX7EHs may or may not be a problem for you with a 30K Ohm load. The only way to know is to give it a try.
I would not bother with getting a matched set, just buy a few extras and make and check to see that the channels are balanced after you install them.
If you need to you can switch them around to match the channels. Also you can adjust the gain and bias for channel matching. Setting the bias counter clockwise increases the harmonics and decreases the gain, going clockwise does the opposite. We usually set these around the middle (12:00 o'clock).
The twin triodes in the 12AX7s and 12AT7s are used in reverse order for left and right in the VTPH-2 so measuring the tube gain could give a different result for a given tube in opposite channels.
Leave the EI 12AT7 (V5) at the output.
Thanks for the note. The smaller plates on the 12AX7EH keep the grid-to-plate capacitance at a lower level than it would be with conventional larger plate 12AX7s (such as the EI and Telefunken tubes). The RIAA ends up being very close with the 12AX7EH tubes on both the four 12AX7 and two 12AX7 units (the 12AX7EH is in the middle capacitance wise). The 12AT7EHs in your unit have the larger plates.
The total effective grid-to-plate capacitance is a product of the gain (mu) of the tube and the actual grid-to-plate capacitance. In this case your 12AT7EHs have less gain (mu = 60), but more plate size than the 12AX7EHs (mu = 100). The effective capacitance is close enough (less than 0.2dB error at 20kHz and this can be corrected with the RIAA trim pots if you wish)
End quote
The tubes I am using are:
original EI 12AT7 at V5
Electro-Harmonix 12AX7 Gold Pin from Tube Depot
Testing : Low Noise and Microphonics
I've compared this preamp to the ARC PH7 extensively and prefer the Herron. The ARC is almost twice the price.