2xel84 rocket

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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Ian444
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Re: 2xel84 rocket

Post by Ian444 »

Within 10% is fine I would think, the mains can vary more than that. Just crank that bad boy up and make some noise!
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M Fowler
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Re: 2xel84 rocket

Post by M Fowler »

I think I will put in a HT voltage switch to go from the

290-0-290 to 260-0-260.
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Deric
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Re: 2xel84 rocket

Post by Deric »

Got a chance to work on this some more tonight....

Tried 2 other rectifiers (2 JJ's and a NOS Mullard). Same results with a all 3...

I'm getting 270-0-270vac from the PT with the rectifier removed. With the rectifier installed and nothing connected but the first filter cap I'm getting 355vdc - lower than expected by 10v or so but still in the ball park. When I flip the standby switch (pre-amp disconnected from the power supply) my B+ drops to 298vdc.

I put a 1r resistor between the rectifier and first filter cap to measure current draw. In standby I get less than 2 mA. When I throw the standby switch I get 80mA. I'm measuring 78mA across the EL84 cathode resistor. I thought that was about about right but the more I think about it - it may be a little on the high side.... Either way, that does tell me that I'm not sinking current somewhere else.

So.....I guess my question for now is: Is my power section drawing too much current or is my PT just not up to the task?

Thanks :?
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marcoloco961
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Re: 2xel84 rocket

Post by marcoloco961 »

Deric wrote:Got a chance to work on this some more tonight....

Tried 2 other rectifiers (2 JJ's and a NOS Mullard). Same results with a all 3...

I'm getting 270-0-270vac from the PT with the rectifier removed. With the rectifier installed and nothing connected but the first filter cap I'm getting 355vdc - lower than expected by 10v or so but still in the ball park. When I flip the standby switch (pre-amp disconnected from the power supply) my B+ drops to 298vdc.

I put a 1r resistor between the rectifier and first filter cap to measure current draw. In standby I get less than 2 mA. When I throw the standby switch I get 80mA. I'm measuring 78mA across the EL84 cathode resistor. I thought that was about about right but the more I think about it - it may be a little on the high side.... Either way, that does tell me that I'm not sinking current somewhere else.

So.....I guess my question for now is: Is my power section drawing too much current or is my PT just not up to the task?

Thanks :?
According to the spec sheet for the el84, w/ 2 tubes in P/P configuration, plates at 300V it's 36ma each for the plates and 4mA each for the screens, so, no, your power section is not drawing too many amps.


Your transformer however is on the borderline. You are drawing 80mA without the preamp tubes, which will add another 10 - 15ma's. Normally the PT should be 1.5 times the max. current draw. Your max. is right about 95-100mA, so a 150mA would be the proper choice. If you are going to play the amp at max. volume for any length of time this tranny will be working hard. It might make it, might not. Some exceed their ratings, while others fall short.

Other than that your B+1 is close enough. You might put a smaller resistor between B+3 and B+4 to raise your last two readings to get them closer to spec. I would give it a go and just keep an eye (or hand) on the PT to see if it is getting hot. If it is, you probably should get a 150mA version. I have never had a tranny even run so warm that it is hot to the touch. Most are barely warm except where the tubes warm them up. If it gets too hot to the touch you will be able to tell right away and should shut it down pronto.
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M Fowler
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Re: 2xel84 rocket

Post by M Fowler »

How are things going with your 18w Rocket projects guys?

Mark
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KT66
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Re: 2xel84 rocket

Post by KT66 »

I'm about done with the layout for mine and am about to order the parts. It's gonna be a reverb version with a few add ons that I haven't tried before like the Watts control that was discussed in another thread and power scaling.
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Last edited by KT66 on Sun Mar 20, 2011 1:06 am, edited 1 time in total.
Ryan

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M Fowler
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Re: 2xel84 rocket

Post by M Fowler »

Ryan,

Very nice but what no dual bright switch :D I just can't seem to build an amp without one must be my decreased high frequency hearing :D

You incorporated a lot of good functions.

Mark
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Deric
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Re: 2xel84 rocket

Post by Deric »

Ryan, Looking good!!

I haven't had a chance to mess with mine much. Still having voltage issues. I've traced thru it a few times and all looks good but my gut still tells me I'm missing something.

Going to play with the dropping string some more.... tried 1k (choke), 18k, 10k, 10k and got closer but not there yet.

In the meantime - here's some pics. Would appreciate any comments!!

[IMG:160:120]http://i40.photobucket.com/albums/e243/stumper1/Half%20Power%20Rocket/th_HalfPowerRocket005.jpg[/img]
[IMG:160:120]http://i40.photobucket.com/albums/e243/stumper1/Half%20Power%20Rocket/th_HalfPowerRocket004.jpg[/img]
[IMG:160:120]http://i40.photobucket.com/albums/e243/stumper1/Half%20Power%20Rocket/th_HalfPowerRocket003.jpg[/img]
[IMG:160:120]http://i40.photobucket.com/albums/e243/stumper1/Half%20Power%20Rocket/th_HalfPowerRocket001.jpg[/img]

edit: Pulled the power supply board and double checked everything (again...). Changed the dropping string to 1k (choke), 15k, 8k2, 8k2. Now my voltages are pretty dang close to the chart in the files section - still just a hair low in PI and the first triode (V1b). I'm getting about 82v on the plate of V1b. May try to lower the 15k just a hair and maybe the last 8k2.

I have some scratching on the volume pot of my guitar. I'm getting a little over .1vdc on the input jack - I assume this is from the low plate voltage. ?? :?:

The PT is getting pretty warm but not too hot to hold onto.

By the way, Matt at musicalpowersupplies.com has been VERY responsive and helpful. He's offered to send another PT to try if I want. Great guy to do business with
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KT66
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Re: 2xel84 rocket

Post by KT66 »

Deric wrote: I have some scratching on the volume pot of my guitar. I'm getting a little over .1vdc on the input jack - I assume this is from the low plate voltage. ?? :?:
If you have DC on the grid like that, something is not right. With the amp powered down and caps drained you could yank the tube and check the socket for continuity between pin 7 and 6 or 8 (probably 8 I think). Nice looking build though.
M Fowler wrote:Very nice but what no dual bright switch Very Happy I just can't seem to build an amp without one must be my decreased high frequency hearing Very Happy
I usually put the dual bright switch in my builds too, just haven't added that on the layout yet. It's funny that you mention it because I have been wanting to ask you if the schem you posted ( shown below) is accurate in that area, as it shows the bright switch toggles between 500p and 100p AND that there is a 100p that is hard wired in and in parallel with whatever value gets switched in. At first I thought this was a mistake, but then I was thinking about it and I like the idea of doing it this way because then you can toggle between 3 different bright cap values instead of 2 and no bright cap which is probably too dark anyway.
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Ryan

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http://Classictubeamps.com
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M Fowler
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Re: 2xel84 rocket

Post by M Fowler »

Ryan I purposely kept the 100pf mica on the volume pot and use on off on dpdt switch for the three way.

But if you take it out and run three caps off a tpdt switch then it would be better switch between 100, 250, and 500pf instead of the way I have it.

Or run 100pf on the volume with 150pf and 330pf on the switch for 100pf, 250 pf and 430pf switching.

I did change some values from what is on the schematic after running the amp for awhile.

Mark
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M Fowler
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Re: 2xel84 rocket

Post by M Fowler »

Deric,

Looking very good. :)

I wired my amp a little different from the layout.

I have 33uf main filter caps then 2k2 (in place of choke) then
(33uf B+2/22k) (20uf B+3/8k2) (20uf-B+4/8k2) (20uf B+5). I did not have 9k1 resistors so that is why I used the lower value 8k2 resistors.

I rewired my amp so that the rectifier is wired to the standby switch then back to the main filter caps. Also using a 5u4 tube instead of GZ34.

Mark
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Deric
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Re: 2xel84 rocket

Post by Deric »

Thanks for the input guys - 8)

Just did some tube swapping and V1b came up to 95v and the PI came up to 175v - these are pretty close to the voltage chart...good enough for me.

Still have a small amount of DC on the input (.08v). Pulled the tube and all checks out fine. Between the cathode and grid I get ~34k which seems right to me (33k grid resistor + 1k5 cathode resistor). I could add a cap to the input ala VVR but it seems like that shouldn't be necessary.

Other than the scratchy guitar volume pot it's very quiet and well behaved.
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soma_hero
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Re: 2xel84 rocket

Post by soma_hero »

Any idea where to get a dual secondary transformer? Doing a project like this soon and would like the option to try it at Rocket voltages or 18W voltages.

Mark and Dartanion, are there more of those transformers available somewhere?

Mark how do you like the amp at both of the voltages? A guy asked me to build him a lightning, but I think I'll talk him into a Rocket instead. Now curious as to what voltage to try at first.
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M Fowler
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Re: 2xel84 rocket

Post by M Fowler »

Soma_hero,

I built a 36w/18w dual voltage switching using Heyboer PT from rjguitars.net

What I like to do is switch to lower voltage on 18w mode it sounds great either way but I like the compression better this way.

Also, liked the option to run GZ34 or 5U4 to lower HT even more and switch between the voltages. This of course can be done on most amps with 3A 5v taps.

It's not for every one, but those that like the EL84 tone will probably find the switching useful. Also depends on pickup/guitar being used as to if you want to engage higher HT.
telentubes
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Re: 2xel84 rocket

Post by telentubes »

I built a lite version of an AC15 a few months ago. Fun, but time to move on. I would like to turn it into a 2XEL84 Rocket and wanted to run a few ideas by you guys.

I had Heyboer build me a PT (M6 steel) which is 280-0-280 at 100ma and 6.3V @ 3 Amps. There is NO 5V tap because the AC15 uses an EZ81 rectifier tube.

1st question... How can I use the EZ81 in the Rocket circuit ? (Mark Fowler mentioned this was a possibility in an earlier post).

2nd question... Will the Allen "TO22" OT I scrounged from another build work in the 2XEL84 circuit? It's 6.6K primary and 4/8 Ohms out. Should be O.K. yes?

3rd... Is there another amp that would be a better choice for conversion? I've looked at the TRex and 18Watt Lite. Or something else that uses an EF86 in V1 as well as the EZ81?
I'm still pretty much a "paint by numbers" builder, but I'm picking up more and more as I go along. I appreciate the help.
Thanks
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