Using a 12at7 in place of 12ax7 in dual rectifier

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MrBoggles
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Using a 12at7 in place of 12ax7 in dual rectifier

Post by MrBoggles » Tue Jan 24, 2017 5:24 am

Just wondering if anyone has experimented with using 12at7's in a dual rectifier..
Either replacing V1,2&3 or replacing just one..
Given they are a lower output,
I'm wondering if they would tame the fizz and gain or just push it back further requiring you to turn up the dials?
Any thought or experiences?
Mesa DR, TC G Maj, Berg F/Sw, PRS Cu24, EJ Strat, Ibanez RG7620 w EMG 707 781, fender HRDlx

bandit2013
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Re: Using a 12at7 in place of 12ax7 in dual rectifier

Post by bandit2013 » Sat Feb 04, 2017 8:50 am

Yes, I have used an NOS (RFT as well as JAN-GE) in my Roadster. I only ventured into the V1 or V2 positions. Never try a 12AT7 in a cathode follower circuit as it will not work well. I was impressed with a NOS JAN version of the 5751 in V2. Actually sounded much better than using current 12ax7's.

MrBoggles
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Re: Using a 12at7 in place of 12ax7 in dual rectifier

Post by MrBoggles » Wed Oct 24, 2018 10:25 am

Thanks for the reply bandit,.. Last year.. Oops.. It's been a while on here..
I ended up changing V12&3 to At7's and it tamed the amp pretty nicely.. It's a subtle change didn't really drop the volume much but tamed the gain a little.. I get a little more range from the gain before it hits the fizz spot..
Still frustrating though.. I've now tried different power tubes, preamp tubes, pickups, volume settings, tone settings, mic placements.. and still when the gain control gets to the point where any sort of usable gain and sustain gets introduced, the high frequency fizz comes in at the same time.. I'm now thinking it must be a bias thing..
Mesa DR, TC G Maj, Berg F/Sw, PRS Cu24, EJ Strat, Ibanez RG7620 w EMG 707 781, fender HRDlx

bandit2013
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Re: Using a 12at7 in place of 12ax7 in dual rectifier

Post by bandit2013 » Sat Oct 27, 2018 9:30 am

I have not done this with the Roadster yet (however I have had done so but did not impress me so I forgot about it), but have with the JP-2C. 12AT7 in the phase inverter position. Another tube you can try out is the JAN/GE 5751. Has more gain than a 12AT7 and is closer to the 12AX7 characteristic just lower gain.

I would also keep one of the tubes in V1 or V2 as a 12AX7 and see if the 12AT7 in V1 or V2 makes much of a difference. Stock tube in the cathode follower position is best. I have tried the Chinese tubes there as well. I actually preferred the character when using a JAN/GE 5751 in V2 and a stock tube in V1 (reference to the Roadster). I think the standard Dual recto is a bit different than the Roadster.

woodbutcher65
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Re: Using a 12at7 in place of 12ax7 in dual rectifier

Post by woodbutcher65 » Sun Apr 07, 2019 5:01 pm

At a certain amount of gain the amp will start sounding fizzy. A lower gain tube will simply allow you to turn the gain knob farther before you reach that fizzy spot.

I've done some tube swap experiments and have certainly heard subtle variations in the overall sound of the amp in addition to the gain changes. I can see how some people have a defined preference for certain specific tubes in certain spots. There is some variety to be be had by doing that.

bandit2013
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Re: Using a 12at7 in place of 12ax7 in dual rectifier

Post by bandit2013 » Sun Jun 16, 2019 11:32 am

The last tube roll I did in the Roadster has me stoked. I have had it this way for a week and I am really satisfied with the end results. No, I did not use the 12AT7 but something similar.... JAN/GE 5751. Have this in V1, Mullard RI long plate 12AX7 in V2 and V6 and all stock everywhere else. This amp never sounded better. Bottom end is tight, top end is more available. Since the first gain stage runs on the high side with a 220k plate resistor when you select CH3 or CH4 (the other recto amps have this plate resistor too) the 5751 helps to cut some of this gain but also adds some brightness to the fist stage. The Mullard in V2 takes advantage of the cold clipper V2B by extending the low end frequency with a slight HF roll off. The same could be said with the Mullard in the phase inverter but it has more of a full tone compared to the Sovtek LPS. I am hearing more control with the presence using the Modern voice on CH3 or CH4 and it is quite musical. the added benefit of using the 5751 in V1 and the Mullard in V2 also helps with the clean channels. Since V1A is first stage and second stage for all channels, the tone stack for the clean channels sits between the first and second stage along with some added warmth from V2A. I will have to get a few more JAN/GE5751 tubes. The one I am using in the Roadster tested much higher than the 12AX7A tubes I have using the Orange VT1000. Typical test results for the Mesa 12AX7 runs from 8 to 9, some of the Mullards 9-10 and the JAN/GE rated at 11 for both triodes, all using the 12AX7 test as there was not one specific for the 5751. To me it is just a number and may not be indicative of the actual gain of the tube as the number is based on other qualities as well.

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