Leslie 147 Amplifier

[oldmacman]

Hey all,

I just traded into a Hammond C-2 and 147 Leslie.

Great sounding rig, but as always, I'm inclined to tweak.

I'm going to rebuild the amp in the Leslie 147 anyways, so I thought I might play with the amp while I'm at it. Has anyone tried to run one of these in fixed bias before? There's no bias tap on the transformer, so I figured I would derive the bias with a circuit like the one in the bottom schematic in http://www.informatik.uni-bremen.de/~da ... a2_all.pdf

Is the small increase in power output worth it? I'd just like to get some more clean volume out of it before it breaks up, and maybe a sweeter and less grinding breakup as well. Any idea on the primary impedance of the output transformer? Would I run an increased risk of blowing up the treble driver?

[billv]

Hopefully I can help a bit.

I would say be careful with the stock V21 treble driver. IIRC, people would regularly damage the phenolic, by cranking up the stock amp, and/or distorting the signal prior to the amp. However, if you clean up the signal, that may help things overall (less clipping distortion components in the signal).

If you change to fixed bias, that may somewhat change the optimal primary impedance that the circuit expects. I'm thinking it would decrease the 'ideal' impedance.

Here are figures from NJ7P for a 6550:
http://www.nj7p.org/Tube4.php?tube=6550

If I'm reading it right, the differences are:
400V plates/300V screens, cathode biased: 4500 ohms; 41W of output
400V plates/275V screens, fixed (grid) biased: 3500 ohms; 55W of output

Whether the impedance mismatch matters is beyond my modest skills.

Sure sounds sweet, doesn't it? I don't know what a 122 conversion does, but it may give a somewhat different tone, possibly without a whole lot of effort.

Anyhow, I have one of these out of the cabinet right now: a 147A from the mid 70s. If you need pics of this one just let me know.

[oldmacman]

The problem with a 122 conversion is that it requires that you change the connector kit on the organ (which is set up for a 147). In addition, it doesn't increase the power at all. Judging by the setups I've played, a 122 has a darker sound as well.

[Bob-I]

In the 70's the organist in my band had a B-3 and 147. I replaced the drivers in the 147 with EV drivers, both high and low and the difference was dramatic. The highs cut like a knife without being harsh, the lows, especally the pedals shook the room.

Personally I'd rebuild the amp to stock and replace the drivers. We used an EVM15B for the low, I can't remember the model # on the highs though. We had to build an adapter for the highs, basically a piece of aluminim stock threaded for the driver, and drilled to mount on the wood.

[skyboltone]

Hey all,

I just traded into a Hammond C-2 and 147 Leslie.

Great sounding rig, but as always, I'm inclined to tweak.

I'm going to rebuild the amp in the Leslie 147 anyways, so I thought I might play with the amp while I'm at it. Has anyone tried to run one of these in fixed bias before? There's no bias tap on the transformer, so I figured I would derive the bias with a circuit like the one in the bottom schematic in http://www.informatik.uni-bremen.de/~da ... a2_all.pdf

Is the small increase in power output worth it? I'd just like to get some more clean volume out of it before it breaks up, and maybe a sweeter and less grinding breakup as well. Any idea on the primary impedance of the output transformer? Would I run an increased risk of blowing up the treble driver?

Gimmie the AO number on the amp and I may already have measurements on the OT primary impedence.

[briane]

Personally I'd rebuild the amp to stock and replace the drivers. We used an EVM15B for the low, I can't remember the model # on the highs thoug

I have an upgraded CV and 147 myself, great combo. Also have a leslie 225 I sometimes pair up.

I would think best to rebuild to stock, they knew what they were doing. No need to redesign.

My 147 was rebuilt by vitagevibe by the earlier owner, though I have yet to go inside to see if they did any mods, though I expect was just a cap job. I love to build a copy someday, and slap that in my leslie 302-c, I'd bet it would really rock!

Generally when people talk about bring a 147 up to 122 specs all they are discussing is going over to the dc relay switching scheme. More complex (need a dc source in the organ, and switching), but does clean up the humm that the 147 is so well known for. Ussually people do not swap out to a 122 amp, though one might assume that. 122 is known for being a bit mellower and more rounded, more jazz rather than rock and roll. The 122 is more highly desired, perhaps as shown only by its price.

Too many projects, so little time!

[briane]

oh, also...If your just interested in adjusting grind level....

Theres a trim pot on the pre-amp in the c2 that does just that.