ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Re: Harp Mods

:

3/13/2002 6:02 AM

Bruce /Mission Amps

Re: Harp Mods

OK, I found that thread on Ted's BBS and I can see why there was a misunderstanding of what he was talking about.
I think what he was saying is, the 12DW7 is an unbalanced dual triode tube, meaning, the two sections have completely different mu, or, the triodes are not in balance with each other.
I don't think the unbalancing of the phase inverter was what he was alluding to, but the fact that since the two sections have such different mu that the amp sounded way different.

Bruce

3/18/2002 1:19 PM

In the tube data sheet I have it listed as a Dual-Triode-Asymmetric. Ik=10ma, VA=250, S=2.2mAV, Pa=3.3W. It's counterpart is a 7247 but it doesn't have the parralleled heater connection so it is a 6.3 300ma. Hey Bruce this kinda reminds me of what we went thru with the WA with the balance difference from side to side. If you guys want to try that I can maybe dig up some mismatched ones for testing. Just let me know,

KB

5/1/2002 1:59 AM
Jim R	Been several weeks since the last post on harp mods, and I am thinking about building a kit- Any new harp weak revelations? Favored speaker setups? Overall karma for harp? Comparison with other commonly used harp amps? Thanks- Jim

7/2/2002 5:14 PM

Geez Jim, looks like you never got a reply! I'd be very interested in this myself.

I'm in the middle of building something similar to the 5E3. I've never heard one but if Bruce went to all this trouble and got such a great response I just have to try it!

I'm using an old Bell & Howell 16 mm projector amp as a base. I recovered the outboard speaker cabinet and am putting the amp inside, keeping all the old hardware for a retro look. I'm using a new chassis and using the old PT. The OT is mounted on the speaker! Near it are some of the filter caps and the field coil is a choke in the negative lead. The connections were all brought back to the amp beneath the projector in a different cabinet by a LONG cable with 6 pin CinchJones plugs. I'll just shorten the cable to a couple of feet to connect inside one cabinet.

The amp had a pair of metal RCA 6V6's that tested good. There's 3 major deviations from Bruce's kit that I'm hoping will work out OK.

The projector had an old 12" field coil electrodynamic Jensen speaker, with the coil doubling as a choke in the pwr supply. I loved the sound of those type of speakers in old 30's radios and I'm hoping it will be pleasing with this circuit. I'm also building it single channel. I'm gonna change the volume pot to 500k since it will no longer be 2 1megs in parallel.

I'm also gonna use a pair of 6SL7's. This will have a bit more gain in the first stage and slightly less in the second. It'll leave me one extra triode so I'm putting in an old Ampeg style trem to tickle the cathode bias of the output tubes. Also, that coil will mean I may have to dink around with that 5K1 resistor, depending on what B+ I get from that old PT.

Obviously, this amp will be real tempting to blow some harp through! It won't be exactly the same as Bruce's kit but still very similar, except for the speaker.

Since your thread never got any bites I guess I won't know what to expect before I'm finished. Hopefully I'll be happy!

---Wild Bill

7/14/2002 4:52 PM

Finished the amp and thought I'd post my impressions of this circuit. What a machine! No wonder it's so popular!

It does come on strong, real fast! That second stage running flat out seems to make the volume on the amp act like a master and the guitar volume the input. I had to drop the gain down on the 6SL7 input triode with a 68k plate resistor 'cuz it was just too much. I also found a lot of oscillation with the high gain input. Tried all the usual things and ended up caving in and putting a 10 pf from plate to grid.

Anyhow, with all that overdriven sound I just had to grab a mike and a D harp. Heaven! It nails that Biscuit Boy/Crowbar sound just cold! I had heard that Biscuit used an SM58 and an old miked Champ on 10.This amp sounds to me exactly like those old cuts.

Wish I had a way to digitize a pix - I'm proud of the look. It's recovered in dark Irish green vinyl upolstery material, with the original grill and the old corners/latches/handle. The chassis is mounted at the bottom below the spkr with a cutout for the brass plate lettered with LectraSet below the chicken knobs. When you open the "suitcase" at the back there's another metal grill backed with the old grill cloth to prevent fingers from poking at the back of the spkr, since there's high voltage there around the field coil.

Should have tried a 5E3 style long ago!

Page 2 of 2