18 watt front with El-34 NFB?

[Ed_Chambley]

I am getting ready to punch the chassis for El-34's.  Other than the difference in wiring up the sockets between EL-84 and EL-34's, the grid resistors for 84 on the schematic I am using the grid resistors are 8.5K and 34's are usually 5.6k.  Will it make much difference?  Also, the OT is 8 Ohms at 4900 and an 18 watter has no NFB. The layout attached is the general layout I am using.  If I wanted to add NFB, how would I go about this?  I attached the layout again and as you can see the grid resistors are on the board and I thought it best to place them directly on the tube.

BTW, here is one in action
26 watt EL34 cathode biased part 2

[sluckey]

Just copy the Plexi 1987 PI and PA.

[Ed_Chambley]

Just copy the Plexi 1987 PI and PA.

What is PA?

[Willabe]

Power Amp. Output section, tubes and OP tranny.

Although some refer to the PA as including the PI.

When I was a kid in my first bands and even now, we say PA for our public address system, for the mics. When I started reading about tube amps I thought why are they talking about a PA for the singers mikes?     

            Brad

[Ed_Chambley]

I still call my Sound System a PA.  What are you supposed to call Mixing Board, Crossovers Mains ans subs amps monitors microphones EQ Vocal Processors Furman Power conditioners snake cable.    I have been a sound man for years and even setup delays for larger outdoor events and all who work with me call it a PA.  What is the 'proper' term?

[Ed_Chambley]

Just copy the Plexi 1987 PI and PA.

By copying the PI and power section, do you mean the wiring and layout or use the same resistors and cap values as well?  I know I need 5m6 on the grids buy a 1987 has 220k resistors feeding the plates whereas the 18 watt has 470K.  Since I am not building a 1987 (I have one) the multi-cap values are much higher than an 18 watt.  I plan to get about 360 or a little less on the plates of the EL-34's.  I am going to use a Dual Rectifier, but am building a octal plug for this and also got an old philips 5v4.  What is the lowest plate voltage you would go on EL-34's?  I have 4 different PT's.  A 550ct, 600ct, 640ct and a 700ct and have a minimum of 4amps on the 6.3 and 5vac.  All will work, but I would like to use the 550 because it is a Mercury and new, but I do have a little concern if using a 5v4 rectifier that I will have enough B+ to get close to 360 on the plates.  I have calculated I will get 24 to 26 watts with the plates from 330 to 360.  I would like to run them lower, but I don't know what El-34's with a plate voltage of 250 to 275 will do?

Sorry, I know this is long with many questions.

[Ed_Chambley]

What is the purpose of the imbalance of the plate resistors on the 1987?

[sluckey]

By copying the PI and power section, do you mean the wiring and layout or use the same resistors and cap values as well?

The reason I suggested using the 1987 circuit is twofold. For one, it has the NFB and presence circuit that you asked for. And two, if you're gonna drive EL34s, you need to use the PI component values for the 1987, not the 18W. Make this part of the amp look just like a 1987. Use the same schematic and component values. Use any layout you want but there are plenty of proven layouts to copy. And if you're gonna use cathode bias, make the necessary changes.

I know I need 5m6 on the grids buy a 1987 has 220k resistors feeding the plates whereas the 18 watt has 470K.

The grid stoppers on a 1987 are 5k6, not 5m6. The orig 18W has 8k2 grid stoppers. Those 220Ks in the 1987 are grid leaks, not plate resistors. The orig 18W uses 470K grid leaks.

Since I am not building a 1987 (I have one) the multi-cap values are much higher than an 18 watt.

I would match the power supply filters to the power tubes, ie, use the cap values of the 1987. It won't hurt anything to use the smaller cap values of the cheaper 18W though.

I plan to get about 360 or a little less on the plates of the EL-34's.  I am going to use a Dual Rectifier, but am building a octal plug for this and also got an old philips 5v4.

Why use a dual rectifier? One 5AR4 or 5U4 can supply all the juice a pair of EL34s needs, especially if you're gonna run low B+. Dual rectifiers are usually used for a quad of power tubes. Using two rectifiers for only two EL34s is kinda like teats on a boar hog. Useless, but may have a 'cool' effect.

What is the lowest plate voltage you would go on EL-34's?  I have 4 different PT's.  A 550ct, 600ct, 640ct and a 700ct and have a minimum of 4amps on the 6.3 and 5vac.  All will work, but I would like to use the 550 because it is a Mercury and new, but I do have a little concern if using a 5v4 rectifier that I will have enough B+ to get close to 360 on the plates.  I have calculated I will get 24 to 26 watts with the plates from 330 to 360.  I would like to run them lower, but I don't know what El-34's with a plate voltage of 250 to 275 will do?

I've only built 3 EL34 amps, all Sunn design, and I ran them at about 500V. I don't know how they will behave with lower B+. But here's an idea... Since you already have a 1987, just temporarily connect the low voltage PT to your 1987. Then you'll know.

What is the purpose of the imbalance of the plate resistors on the 1987?

It is to compensate for the unequal gain of the LTP circuit tubes. The idea is to provide balanced (equal) drive signals to the power tube grids.

[Ed_Chambley]

I plan to get about 360 or a little less on the plates of the EL-34's.  I am going to use a Dual Rectifier, but am building a octal plug for this and also got an old philips 5v4.

Why use a dual rectifier? One 5AR4 or 5U4 can supply all the juice a pair of EL34s needs, especially if you're gonna run low B+. Dual rectifiers are usually used for a quad of power tubes. Using two rectifiers for only two EL34s is kinda like teats on a boar hog. Useless, but may have a 'cool' effect.

What I am saying here is not really a Dual Rectifier, it is 2 different rectifiers.  I am going to make a ss rectifier which I can plug into a single octal socket.  Remember, you explained to me the convenience of breaking an old tube and building the ss inside of it  that way I can easily swap them.  Why the 5U4 over the 5V4.  The 5v4 needs less heater amps, has less voltage drop.

EDIT... untangled quote... sluckey

[sluckey]

Why the 5U4 over the 5V4.  The 5v4 needs less heater amps, has less voltage drop.

No reason. I pulled that straight outta my back pocket to use as an example. It's a moot point since you're not gonna run parallel rectifiers. 5V4 is fine.

[Ed_Chambley]

. And if you're gonna use cathode bias, make the necessary changes.

I am loosely following the layout originally posted by you.  Can I use the cathode resistor values for EL-34's or will I have to make changes.  This is from part of your reply #7.

[sluckey]

Who are you quoting?

[Ed_Chambley]

Who are you quoting?

I am quoting a portion of your reply.  #7
Here is the whole thing:
The reason I suggested using the 1987 circuit is twofold. For one, it has the NFB and presence circuit that you asked for. And two, if you're gonna drive EL34s, you need to use the PI component values for the 1987, not the 18W. Make this part of the amp look just like a 1987. Use the same schematic and component values. Use any layout you want but there are plenty of proven layouts to copy.

And if you're gonna use cathode bias, make the necessary changes.
This section I wanted you to elaborate on.  Will the layout cathode resistors be sufficient?

[sluckey]

Will the layout cathode resistors be sufficient?

I don't think so. The 18W uses a single 125Ω/5W cathode resistor. The only P/P cathode biased EL34 amp I'm familiar with is the Matchless Clubman. It uses a 270Ω/15W cathode resistor, one for each EL34. You could use a single 135Ω/big wattage resistor. Allow some extra space on the board if you're gonna use the 18W TMB layout.