Designing for 12AT7 preamp

[Bob-I]

A friend of mine wants an amp built for acoustic electric guitars, clean all the way to about 30 watts. He also likes the sound of 12AT7's and EL-34's. My thought is to use a Fender AB763 type circuit, dropping the normal channel and trem. Topology would be:

12AT7 gain stage > FMV tone stack > 12AT7 gain recovery > 12AT7 reverb driver > 12AX7 reverb recovery > 12AT7 LTP PI > EL-34 output. I'm a bit puzzled by why Leo added that additional gain stage after the reverb recovery, but used a voltage divider to essentually get 0 gain.

I could leave the first 2 gain stages as they are in the AB763 but changing Rp and Rk for 12AT7 might optomize the amp for what he's looking for.

Thoughts?

[selloutrr]

you might like the fet input option of the dumble if he is using an acoustic electric pickup.

the AU7, AT7 and AX7 are interchangeable in most gain stages.

[Bob-I]

you might like the fet input option of the dumble if he is using an acoustic electric pickup.

the AU7, AT7 and AX7 are interchangeable in most gain stages.

Yes, I know that but I've read that 12AT7 are better optomized with lower plate resistors. I know Fender went to 47K on the PI later to try to clean up the PI (dummys at CBS)

My friend doesn't want any SS components in the signal. I suggested the FET board and he rejected the idea.

This friend has done me some solids in the past. Most recently he did a georgous wine red matte finish on a P-Bass body for me. Gotta take care of your friends!!

[selloutrr]

Just a thought
consider,

metal film resistors. as well as replacing any ceramics you might use in the design with orange drops (a studio mod for silence).
Buy hand selected tubes for balance and microphonics (this might be the most important to keeping the circuit smooth and quiet)
over filter and over spec the transformers so they never reach the stress level of there design.

also consider working in an Ampeg Reverb a graphic eq or parametric eq with a high and lo pass filter. I think that's wicked awesome on acoustic for knotching out the low end rumble, weird 315-500 honk, and brittle high end of most cheap OEM and pop in pickups.

[FunkyE9th]

Maybe look for a tube recording preamp and a HiFi power amp schematic?

[Bob-I]

Just a thought
consider,

metal film resistors. as well as replacing any ceramics you might use in the design with orange drops (a studio mod for silence).
Buy hand selected tubes for balance and microphonics (this might be the most important to keeping the circuit smooth and quiet)
over filter and over spec the transformers so they never reach the stress level of there design.

Good points, and already considered. I'll most likely use CF for signal and MF for all other resistor, 6PS caps or Mallory 150 for coupling and tone caps, SS rectifier and upping the first stage of filter for quiet, I did that on another amp I built for him and it worked out great.

I have a small stash of NOS RCA and GE 12AT7s so I'll pick from that batch, cost is also a factor. Xformers I like are the Allen amps Heybour. He has a nice overspec'd Princeton PT that'll supply the EL-34's heaters just fine but still keep the B+ at about 400V. He also has an overspec'd Deluxe OT that should handle the 30 watts just fine.

also consider working in an Ampeg Reverb a graphic eq or parametric eq with a high and lo pass filter. I think that's wicked awesome on acoustic for knotching out the low end rumble, weird 315-500 honk, and brittle high end of most cheap OEM and pop in pickups.

Actually this guy designs and builds his own pickup system that avoids that problem. He wants this amp to demo his guitars. I'll take a look at the Ampeg reverb, thx.

[Bob-I]

Maybe look for a tube recording preamp and a HiFi power amp schematic?

Good idea.

[selloutrr]

becareful as most tube preamps for studio use add alot of color by design so if you are going for clarity to demo a pickup it's not going to be a very true listening station, pretty but not true to the pickup.

really if it's for a demo he should use a cheap run of the mill amp anyone can buy from any music store to show that the average joe can dial in great tone.

We don't mix on NS-10's cause they sound good

sounds like you have your mind pretty much made up happy building let us know how it turns out

[Bob-I]

becareful as most tube preamps for studio use add alot of color by design so if you are going for clarity to demo a pickup it's not going to be a very true listening station, pretty but not true to the pickup.

really if it's for a demo he should use a cheap run of the mill amp anyone can buy from any music store to show that the average joe can dial in great tone.

We don't mix on NS-10's cause they sound good

sounds like you have your mind pretty much made up happy building let us know how it turns out

I agree with all of what you said. He's trying to market an amp as well. He has a generic PA and some amps as well. I built him an amp awhile ago and he's having trouble selling it because it's too simple, one knob tone and no reverb. Taking that feedback he wants a full FMV tonestack and reverb.

[Merlinb]

Since many electro acoustics use peizo pickups, you'll probably benefit from a very high input impedance, by bootstrapping the grid-leak at the input of the amp.

[flood]

i wanted to build a tube preamp/poweramp combination for acoustic guitar, and was considering something along the lines of an ECC88 SRPP input stage. how would that work?

[Bob-I]

Since many electro acoustics use peizo pickups, you'll probably benefit from a very high input impedance, by bootstrapping the grid-leak at the input of the amp.

Actually this is for a PU system that my friend developed and markets. The input impedance is 1M so that's a no brainer.

[selloutrr]

no offense Bob but for someone asking for ideas and help you very condescending in your replies.

[benoit]

no offense Bob but for someone asking for ideas and help you very condescending in your replies.

I read a coupla times to make sure and I don't get that from the replies. The both of ye (in my experience on the forum) are good fellas and have contributed lots to the forum. It'd be a shame have a blowout a-la the "HAD is policing Ebay" thread over a perceived slight. It's the internet, and it's way too easy to sound like a jerk when you don't mean to at all.

[selloutrr]

i'm not trying to fight at all!
I'll stop posting on this topic
best of luck on the build i'm sure the client will be very happy!