EL84 Blues Amp

[10thTx]

The sound board material is a nice quality 5/8" plywood covered with Oxblood grill cloth with the gold thread in it. The wood is sappele with crotch mahoganey book match panel and flamed maple trim.

I'm using a Cannabis Rex speaker in it which is a great match for the amp for the tone I'm looking for. It sounds pretty good with an Emminence Red,White and Blues also.

& These are the guitars I built.

http://s28.photobucket.com/albums/c216/ ... Tele-1.jpg

http://s28.photobucket.com/albums/c216/ ... uitars.jpg

With respect, 10thtx

[novosibir]

Maybe you've wired it in the amp different, but the cathode switch - as drawn in the schem - is w/o any effect! The cathode bypass caps of the EL84 in series w/ each a 100K to ground don't influence anything.

The R's shouldt be directly in series w/ the caps to ground, for anti pop purpose, when the switch is switching the junction between the caps & the resistors to ground!

Larry

[novosibir]

Ah, you've posted another schem a minute before I posted, where the switch now is removed.

But another prob is, that you won't hear much of the reverb, because the dry mix R between R & R2 of 150K (former 100K) is way too small.

You'd at least need a 2.2M there, and a smaller Reverb pot, to come to another voltage divider ration in conjunction w/ the 2.2M! But then you'd need an additional recovery stage, to push up the loss again.

Larry

[10thTx]

Can you take a look at this and clarify what you're saying, please? I am probably missing something obvious but I'm seeing the cathode cap and resistor in series to ground.

http://www.el34world.com/Forum/index.php?topic=4437.0

I'm not sure I am following you??? I can easily edit the schematic and layout but I'm not sure what you're advocating I do different? I have used the switch with 1/2 & 1/4 power and it worked great.

Any chance you could provide a schematic of how you would wire it?

Regarding the reverb ...... it works fantastic! I have owned and played Fenders for years and I prefer this reverb. I play it on around 3 to 5 and have all the reverb I will ever need. I usually had Fenders set on 2 to 3. Above 5 is too intense for me.

Did you happen to notice the value of the reverb pot? Fender is 100k & I use 1M or 2.2M. I have the dwell set at about 1/2-2/3. I initially tried a 1M resistor instead of the pot and it was too intense. I also tried a 2.2M reverb pot and lowered it to 1M on two of my 3 amps because it was also too intense.

A similar reverb has been used by some of the D'Lite builders which is where I borrowed (stole) the idea and tried some different values.

I view it a very successful reverb if you prefer more mids verbed. I have put in on quite a few amps.

With respect, 10thtx

[Andy Le Blanc]

it doesn't look familiar thats all...... but the verb universe deosn't have to
revolve with fender at its axis..... you could go one futher even.....
source the drive (R) from the top of master ( since you already have a dwell)
maybe put a push-pull on the master...... so you can turn down the dry signal
and go surfing.....

[Luthierwnc]

Forgive me if I missed this in the other replies but a few things jump out. One is that 2k2 screen resistors are a bit stout. You may also need to put an anti-pop resistor across the PAB switch. I have a non-HRM (dpdt version) that has a 350k across it just so the brightness doesn't peel the skin off the couple at the front table.

You may also consider a post PI cut. Again, knocking down too much brightness is a big tool on EL84 rigs -- that's why most of these amps have them. It doesn't have to be a panel switch -- you can hardwire it to taste. Finally, the 100k series resistor around the reverb exit and entry is like some "D" amps. You might just throw a 1m trimmer in series with the 330k resistor coming from the reverb so you don't overwhelm that stage. Some of the guys on the Dumble have also used ECL82's or ECL86's for a little more throw from a single 9-pin. I have the parts but am waiting for the amp

Don't scrimp on the speaker. Skip

[novosibir]

Can you take a look at this and clarify what you're saying, please? I am probably missing something obvious but I'm seeing the cathode cap and resistor in series to ground.

http://www.el34world.com/Forum/index.php?topic=4437.0

I'm not sure I am following you???

The cathode switch in your schem isn't wired, as the one on Hofmann's site!

On your schem both the minus sides of the electrolytics are going to each a middle contact of the DPDT - and from each one of the left contacts of the DPDT each a 100K is going to ground - the right contacts of the switch aren't connected anywhere.

Now switch, that the middle contacts of the DPDT are shunting to the left contacts, then you have each of the electrolytics in series w/ its 100K to ground - you won't hear any effect of this!

Now switch, that the middle contacts of the DPDT are shunting to the right contacts (which are n/c) and nothing will happen

The 100K's have to be hard wired to the minus side of the electrolytics, the conjunctions to the middle contacts of the switch, and the left contacts (or the right ones) directly to ground.

The 100K's are only for pop suppressing purpose.

Larry

[novosibir]

Regarding the reverb ...... it works fantastic

I've never tried a reverb wired this way, but I couldn't imagine, that it will work w/ just a 100K (or 150K on the other schem) between the Send & Return of the reverb area. It's been just my thoughts.

But if it works (as you say), I can imagine, that the reverb easily starts to oscillate at some control positions, because it seems, that due to just a 100K (150K) between the reverb's input & output the output probably is feeding back into the input (i.e. when the dwell is dimed)?

Do you have an issue, like I've described?

Larry

[skeezbo]

10thTx:
Thanks for sharing!

I have followed the development of this circuit over at the Hoffman site. I, too, am trying to design a perfect blues amp, and you have always been a few steps ahead of me with your interesting innovations. I have two questions:

Early versions had a cathodyne inverter. You must have been able to compare that style with the LTP in the same circuit, something I've always wished I could do. What are the sonic differences? Why did you choose the LTP?

The thread on the cathode bias power reduction at Hoffman's site was a fascinating read. Why did you choose the simple 1/4 switch over the 1/2 to 1/4 variant or the more variable pot attenuation?

Thanks!
Skeez

[10thTx]

Larry,

I think you asked a fair question about oscillations. I don't know if I have ever turned the reverb pot to 10, so I just tried it. I used my guitar at "8" and I turned the volume of the amp up actually louder than I play it with the band (keyboard, bass,drum, acoustic/electric and 3 singers).

NO oscillation problems with reverb at 10 and 1MA dwell pot at 8. Just a ridiculous over the top amount of reverb. I did not find any of the fenders useable past 5 for me either. As I previously, mentioned I prefer this over the Fender reverb. I have not experienced any problems with it on any amp I have used it on. It is usually set on 3 and maybe bumped to 5 for a slow blues tune. So the reality is that it provides waaaayy more reverb than I will ever use just as the Fender did.

I'm not asking anyone to ditch the Fender type. It was just a concept amp so I stuck the type of reverb in that I like the best.

Skeezbo,

I have pretty limited electrical knowledge so instead I have just experimented like crazy with amps to find what works and what I like.
For example I took the Carolina Blues Special amp that I built and did my best to compare everything being the same except the LTPI vs. the concertina PI. I can't say one is better than the other, IMO just different.
The LTPI is louder and seems to have more grind. Maybe the concertina is more relaxed.

Regarding the switch being just 1/4 power. The one I put on a friend's was 1/2 & 1/4 and I thought that I probably would use the 1/4 but not the half power feature. No particular reason other than that.

Guys ..... I don't plan to build this amp. I was just trying to return some of the generousity of sharing ideas that has been extended to me. I'm quite happy with the 6V6 version I have.

With respect, 10thtx

[10thTx]

OK, I tried my guitar with humbuckers on 10, dwell on 10 & reverb pot on 10 with my amp louder than I play with my band .......... NO oscillations. However that has to be 2-3 times beyond the max of the reverb I will use.

I may be wrong, but my guess is some of the high gain amps would oscillate if everything were dimed?? I think sometimes it's just about designing a good tone without problems at the limits that you want.


With respect, 10thtx

[sliberty]

Usually, oscillations are designed out of an amp before it is offered commercially. Often this is done with good lead dress, but in some cases with added resistors and/or caps to reduce the frequencies of the oscillation.

In my own hobby amp building experience, a residual oscillation annoys the hell out of me even if it is only present when the amp is set in a manner that I wouldn't normally use - maybe I am just anal in that way. My JTM45 had a squeel when cranked and set very bright. I was told that it was typical of the model, even from Marshall, but that wasn't acceptable to me.In that case, it turned out that roof flashing in the bottom of the head cab did the trick - whew!

[10thTx]

Larry,

I've missed the obvious before, so please be patient with me. In this schematic, I have wired and used the one on the right with full, half and quarter power. It does work just fine.

PLEASE explain again why the one on the left "won't" work. I am trying to understand what you're saying but not getting it?

As I am seeing it, both switches will disengage both cathode caps to ground to get 1/4 power? What am I missing?

With respect, 10thtx

[sliberty]

As I see it, the one on the left won't do much because when the cathode caps are connected, they are in series with large resistors. This 100K resistance is so much larger than the normal 270R cathode resistor that all current flow will simply go via the 270R resistors instead. In otherwords, it will be as though the cathode bypass caps aren't even in circuit.

I am not really sure what the purpose of the 100K resistors would be in the one on the right, but notice that the center tap on the switch goes directly to ground. So when one of the cathode bypass caps is in circuit, it is connected diretly to ground as it should be.

[10thTx]

OK, mayhe I drew the switch wrong. However I DID wire one up sometime back and it worked .......... however I wired it?

I don't understand why it worked, but here are some comments that may shed some light on why it works OR what is incorrect in how I drew up the schematic?

So tell me please how this works? Answer/reply below.....

Well, if we don't use a common cathode resistor, then there will be negative feedback at the cathode due to the current flow during a signal. We would have to bypass each resistor to eliminate the negative feedback and have full power. If we did not bypass the resistor, the negative feedback due to the resistor reduces the current swing through the tube due to signal, but that also reduces the voltage swing at the plate of the output tube, because the voltage swing is caused by a current swing through the primary impedance of the output transformer.

In the end, less current swing times less voltage swing equals less power output.

What about the bypass caps? If we bypass, we get full power. But what's up with the funky effect of connecting the negative ends together??

If the negative ends of both bypass caps are grounded, are they not effectively connected together? Yeah, they are connected to true ground too; however, if you disconnect the negatives from ground, but keep them connected to each other, they have a "virtual ground" at their junction and have the same end effect. In fact, 2 caps connected in this manner are essentially in series, and the net value is half of the value of each individual cap. The pair of (maybe 22uF) caps could be replaced with a single 10uF cap. This idea (a pair of bypass caps being replaced by a single cap of half-value connected between the 2 non-grounded points to bypass) has been used before by Quad amps and was explained by Morgan Jones in Valve Amplifiers.


After reading this can sometime draw and post a DPDT switch (in the style I drew the DPDT) and show what would be correct? Again, I installed this on a friend's amp and it works. There are a number of individuals who report this works also. Unfortunately I did not keep a schematic or photo of what I did.

With respect, 10thtx




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