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Dirty-South Blues Harp forum: wail on! >
Try this weird experiment for amps 2XEL84 & 2X12A?
Try this weird experiment for amps 2XEL84 & 2X12A?
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LittleJoeSamson
376 posts
Jul 28, 2010
11:31 PM
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So, I thought I would give Blanche, the vintage Ampeg Rocket a rest. It's definitely got THAT tone with the Jensen original speaker, and I have retubed it with great Sylvania 6SL7WGT's, a hot CBS 6V6GTY paired with a Raytheon Black glass VT-107A and the Matsushita 5CG4 rectifier. The USN GE-6087/5Y3WGTB is also good.
Anyway, the Blackheart BH15H piggyback has sorta taken a back seat, or as backup amp. Still, it has a real good tone and Chicago sound. Just has not had the cojones to cut thru a full band mix.
As I have detailed previously, I have rolled tubes on this with satisfaction. I got a great pair of black plate Sylvania 6BQ5's labeled Baldwin that provided punch and texture. Have tried various combo's of pre-amp tubes in V1 & V2. 12AX7's/7025's/12AT7's/6829's/12AV7's/5965's/6201's/12AY7's/6211's/12AU7's/5814a's.
Some were good, others not so. The 6211 & 6829 was great for tone, but not much volume. 5814a & 12AY7 was nice and fuzzy. The 5965 and a 12AT7 Sylvania Gold Pin was surprisingly good. Any combo's using even quality 12AX7's or 7025's were way too bright and prone to feedback.
SO, then I went to a real oddball: a 12DW7/7247 ...a dual triode with one side rated as a 12AX7 and the other a 12AU7. Mismatched.
I paired this with all various combo's. The 6211 and 12AY7 sounded best in V1 with the 7247 in V2. The odd pairing of the 7247 in V1 and a 6829 in V2 sounded even better.
BUT, I have another Mullard 7247 handy. This was what I tried tonight: two 7247's in pre-amp... V1 & V2.
I set it up, using one of the cheap karaoke mic's I recently picked up, and mic'ed the cab.
MAN...the unit barked and honked with a classic Chicago tone @ 15 watts and absolutely NO feedback issues.
Had no trouble swimming upstream against other instrument volume. Could have skipped micing the cab.
Had numerous compliments tonight, from audience and other players.
For those of you with this tube layout, you might want to try this.
Pro Jr. owners ?
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5F6H
267 posts
Jul 29, 2010
1:51 AM
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The Blackheart BH15 has what's called a "long tail pair" phase inverter (LTPI). This means that one half of the LTPI delivers the signal to one power tube, the other half of the LTPI delivers the signal to the 2nd power tube in the push-pull configuration. With a 12DW7/7247 in V2 you are unbalancing the push-pull effect (making the amp work more like a single-ended amp, like a champ)...similar approaches have been used in harp specific amps.
The effect is usually that fidelity is reduced some, which can serve to warm the sound & take off hard edges, because the push-pull output section is running less efficiently, you can get more sweep on the volume knob.
This can be tried in any amp with a LTPI - Pro Jr (as mentioned), Blues Jr, Laney LC15, any 6L6 powered, fixed bias Fender (& BF SF deluxe/deluxe reverb). It's possible that increased current draw on the low gain side of the tube might possibly burn out a plate resistor in amps with 1/2W resistors, but this is relatively rare & the BH15 runs relativey low voltage & uses 1W resistors, so should be pretty safe. The effect varies varies from amp to amp, but it's definitely worth having one on hand to try.
In amps like the 5E3 Tweed deluxe, tolex Pricetons & other amps that use a "cathodyne" phase inverter (Peavey Classic 30, Silvertone 1432, 5E4/5E5/5E6/5E8 Fenders), a 12DW7 will just alter the gain structure (as it does in V1 of the BH15...as it would in V1 of a tolex Fender), but not cause any imbalance to the output. Still maybe worth trying.
In V1 of a 2 channel tweed Fender amp, a 12DW7 gives a different gain structure to the Normal & Bright channels...which some might find useful if you pack mics with big variances in output (especially if the "Bright" cap is removed so both channels are voiced more similarly).
Last Edited by on Jul 29, 2010 6:36 AM
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LittleJoeSamson
378 posts
Jul 29, 2010
1:23 PM
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Thanks for the technical input, 5F6H. You stated it concisely, and better than if I had tried.
You're right...it has more of the sound and tone of a classic Champ now.
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LittleJoeSamson
379 posts
Jul 29, 2010
3:34 PM
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As a sidebar serendipity...I pulled the other 7247 out of my 120 watt Traynor PI slot. I replaced it with what I thought was a Sylvania 12AT7, and tried it out with one of the recently acquired karaoke mic's . It sounded better than before ! I could now crank up volume in both channels and turn the master up, too. It sounded so nice, I thought I would compare it against a 6829 I had employed before. Upon removing, I found that it was actually a 12AY7. Testing the 6829 was good, but not as good as the 12AY7.
Sounds terrific with bridged channels and effects: Reverb on 1, Tremolo @ 9 intensity and 6 speed.
Now I need a roadie to haul 85 pounds around !
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