Tube Amps / Music Electronics
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|7/12/1999 6:32 AM|
||Opinions on a 5F2A-5E3 combo|
Recently I was trying to decide whether to build a 5F2A Princeton or a 5E3 Tweed Deluxe as my next DIY amp project.
At first I thought I would build a 5F2A with a Hammond 125E Universal OT on a large chassis. When I grew tired of it I could hack it up and reuse the power supply section, OT, chassis and tubes to build a 5E3. Then it occured to me that with the right switching I could share a single power supply section, OT and speaker between the "guts" of a 5F2A and a 5E3. Since these are some of the most expensive parts the difference in cost between a straight 5E3 and a combo 5E3/5F2A would be less than $50.
Here's my plan. Build a 5F2A but cut it off at the two wires going to the OT, the negative feedback from the speaker, the B+ at the first filter cap after the rectifier and the filament supply. Do the same with the 5E3 (the connections to the OT are slightly different). Use just one OT/speaker and power supply section. Insert a heavy duty, seven pole, two position rotary switch. The switch would switch the filament supply, B+, OT primary (three wires) and speaker feedback. The switch would have a "Princeton" position and a "Deluxe" position.
When one mode is selected the other mode is completely unpowered. I plan to populate all of the tube sockets so I can switch modes by simply flipping the switch.
The mode would be switched only when the amp is turned off. A rotary switch is prefered over individual switches since I want to avoid, for example, applying the B+ without also supplying the filament. An alternative to a rotary switch would be to gang together a group of DPDT switches.
I plan to follow the usual modern construction practices:
- star grounding
- add a standby switch
- use a twisted pair of wires for the filaments.
I know this is a weird idea for an amp but for me part of the reason for doing a DIY amp is to do things that normal amp companies wouldn't try.
Here are my questions:
1) Is the rotary switch a bad idea? Am I asking for problems with hum by routing filament and unfiltered B+ through a single switch? Is the rotary switch going to cause reliability problems? If the switch ever failed and shorted out or left connections open I could quickly fry some tubes.
2) Driving the filament for all three 6V6s and three 12A*7s is only 3.3A (plus the pilot lamp). If I choose the right PT I wouldn't need to switch the filament supply, which would simplify the switching. Would it be a bad idea to always supply the filament to the unused tubes?
3) Is it okay to hook the inputs to the two amps in parallel (so a single input jack is used for the two modes)? There would be one 1M resistor with two 68K resistors, one going to the grid of each of the two preamp tubes. I assume a few 100 mV going into the grid of an unpowered 12AX7 won't hurt the tube. Will the load introduced by the unpowered 12AX7 affect the tone in the powered mode?
4) Does the negative feeback in the 5F2A need to be switched on/off or can it always stay connected (i.e. is there any harm in running a few volts into the cathode of an unpowered tube)?
5) The amps are similar in a lot of ways - same tone control, same power tubes (albeit different configurations). I've never played a 5E3 Deluxe and I wonder if these are going to be too similar tone-wise. Once I go through the trouble of building this am I going to find that one amp is just a slightly louder version of the other?
Any comments will be appreciated.
|7/12/1999 10:01 AM|
Here's an idea that's easier, cheaper and safer:
Build the Deluxe, but replace one of the power tube's grid resistors with a pot of the same value, wired as a volume control. This will provide a continuous variation from push-pull to single ended.
Use the negative feedback scheme from a blackface Princeton, with an SPST switch added that shunts the 47 ohm resistor in the cathode circuit of the second preamp stage. This is your on-off switch for negative feedback.
Not quite exactly the same thing, but probably close enough, and maybe more useful.
|7/12/1999 9:57 PM|
Great idea on using the pot in place of the grid resistor! I had been racking my brain on some kind complex power supply/dual output transformer setup to run an SE / push pull combo. It's funny how easy it is for me to sometimes overlook a simple and more effective way to design something in favor of trying to brainstorm some kind of over complex monster!!!
I guess that's part of the learning process.
|7/12/1999 5:00 PM|
|Mark Hammer||Re: Opinions on a 5F2A-5E3 combo|
I have had a 5F2A for the last 23 years, and it has never let me down. It is the finest harmonica amp I have ever heard in my life (no small thanks to the 8" JBL I put in it), and always sounds the way I want it to sound with my guitars.
People hold the same opinion about Deluxes of that era.
So, my short response is "Why bother?".
Having said that, there are plenty of things you can do with a straightforward 3-tube Class A amp. From time to time, I switch rectifiers (5Y3 to solid-state and back again), switch output tubes (6V6 to 6L6), and disable the negative feedback. (NOTE: Instant icecream headache tone can be achieved by sticking in the diodes instead of the 5Y3, sticking in the 6L6 and lifting the negative feedback, and making sure you use bright single-coil PU's. Ouch!). I've never seen a single EL34 class A amp out there, but there should be some way to tweak this baby into using an EL34 instead of a 6V6 or 6L6. The possibilities are not endless, but they are more than sufficient for many uses.
Build the Princeton. Once you've had Class A, you'll find it hard to switch back.
(extra advantage: no more output tube matching)
|7/12/1999 5:40 PM|
I just finished a homebrew 5F2A and it is most excellent. I'm running mine with an EL34 and a solid state plug in, though it can run a 6L6 with the plug in or a 5Y3 rectifier and a 6V6. Haven't tried the 6V6's yet(need to get a 5Y3) but love the EL34 in there. The 6L6 doesn't seem to overdrive as nice as the EL34 to me. It's hard to believe that so much good tone can come from such a simple set-up(one pre amp tube & one power tube) but it does. It can get right loud too.
Go for the SE class A, I think you'll be amazed.
|7/13/1999 4:20 PM|
Hi, Jason. Small world; I just finished a 5F2A clone myself. It is in a pine box with one alnico 12" speaker. Like you I wired for the ability to use different power tubes and found by far that the EL34 is the best. I have a Telefunken 12AX7 and a Mullard EL34 in it and it just kills, by far my favorite amp. The EL34 was much smoother when overdriven than the 6L6, and different brands of EL34 sound way different.
I added a feedback level pot to the amp, basically a 100K pot in series with the feedback resistor. Not only does this change the gain, it also changes the tone. Less feedback=more gain and less bass. More feedback=more bass, less gain and a cleaner tone. I also added a triodepentode switch like on the AX84, this allowed full overdrive at even lower volume levels. I haven't messed with rectifiers very much, the amp currently has a 5Y3. The amp is a fantastic recording amp for clean or overdrive, and it has the magical quality some of us seek: I can play it turned all the way up and the wife doesn't complain! I hope to get some pictures and maybe some sound files on the net in the next few weeks.
|7/15/1999 1:42 PM|
I finally got to go jam with mine last night. It sounded great! It gets right loud! What is the B+ in you amp? Mine is using a solid state rectifier giving me about 389vdc on my first filter cap. Just out of curiosity what value dropping resistors did you use in your power supply. I found it real difficult to get the plate and screen voltages in a good range for both the 6L6 and an EL34. Right now I have 2k2 and a 22k for dropping resistors which seems to be more suited to the 6L6 with a plate voltage of 378vdc and a screen voltage of 376vdc. When I insert an EL34 with the same dropping resistors my plate voltage is 380vdc and my screen is 360vdc. 20 volt difference! It sounds good but would probably sound better with the screen a little closer to the plate voltage. I think if I optimize it for an EL34 running a 6L6 goes out the window but at this point I don't think I care to run a 6L6.
Sorry for the long ramble.
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