Pentode (EF86) pre-amps and combo's?

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AC Cobra
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Re: Pentode (EF86) pre-amps and combo's?

Post by AC Cobra »

Will do. The 4th guitar on the list is mine (provided it dosent get bumped back by more orders) I also need to free up the CNC so I can cut out my chassis, or use one of the mills at my day job.
Thanks for the encouragement.
There will also be a video of the 2 guitara we are building at this moment, but it will probably be recorded with amplitube so not entirely revalent to 18 watt.com
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Re: Pentode (EF86) pre-amps and combo's?

Post by dotfret »

I noticed this a while back, but forgot to mention it. If you look up the data sheet for the Z729 (which is GEC-MOV talk for EF86),

http://www.mif.pg.gda.pl/homepages/fran ... 29_GEC.pdf" onclick="window.open(this.href);return false;

there is a section on af operation with lots of handy hints on how to minimise hum and microphony. Particularly, you will find there is a sentence suggesting that connecting the screen to ground via a paper capacitor of at least 0.1uF stops a lot of noise.

Worth a read.
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Re: Pentode (EF86) pre-amps and combo's?

Post by JMPGuitars »

Huh. Apparently I never posted my results. The RFT EF86 tubes are excellent. However, there are plenty of fakes out there, so be careful if you're looking for an RFT tube.

Here's a couple demos with an RFT EF86 in a combo:


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Re: Pentode (EF86) pre-amps and combo's?

Post by Bieworm »

F***in great tone Josh!!!
When did you record this?
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Re: Pentode (EF86) pre-amps and combo's?

Post by JMPGuitars »

Bieworm wrote:
Mon 04/15/24 1:46 am
F***in great tone Josh!!!
When did you record this?
lol, thanks. Uhhhhh, 9 years ago. Somebody liked one of my posts so the thread pooped up for me. 🤣
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Re: Pentode (EF86) pre-amps and combo's?

Post by dwetherby »

Hey so I don’t know how to post on this but I bought this amp from this guy for like 150 and sometimes it just sputters and gets a tone drop I found a cold sodder on a preamp tube, I fixed it and don’t get a drop out but just sounds kind of muddy. I’m also wondering about the resistors on the tubes and how schematics I look at don’t have the resistors going off to the board but they jump pins??? Also all the schematics I see are 3 ax7 but this one has two? I know nothing but to discharge the caps to not get shocke. Does this look right to you guys? Could be a tone pot issue possibly? Guy I bought it from said he built it off this site.

Also this is all at volume at about 3 (can’t tell if gain is volume or volume is volume…kind of all sounds the same) to keep up with band..at low levels it sounds great up to 2

Edit:
Just added two pictures of the pots for volume bass gain and Treb…I feel like my other amps I have and other amps I’ve seen don’t have resistors and stuff on the pots? A resistor on the 1/4” input??? Does this look right or excessive? Sorry and thank you in advance
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Re: Pentode (EF86) pre-amps and combo's?

Post by Daviedawg »

Probably a single channel 18 w. Hard to tell from the pics which one but you can look at the downloads section to see if you can match any with the values you have there.
The build style is unusual with the separate faceplate and chassis. My big question is has the faceplate been grounded? The green ground serves the pots but I see no dedicated ground back to the chassis.

Dd

Edit: the fitting of components directly to pots, switches and other parts is normal in wired assembly. PCBs commonly have those components on the board not on the pots etc. Increasingly common is for the pots etc to also be on the board which leads to lots of failures as the components move in use while the board is stiff so eventually the legs or tracks can break.
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Re: Pentode (EF86) pre-amps and combo's?

Post by Bieworm »

The resistor on the input is the 1M grid leak resistor from the 1st amplification stage. It is really necessary, and more often than not right there where yours is.
I do see some odd stuff in your amp, apart from the poor soldering job all over.
The input jack is a switching type, but it’s not grounded at the switching. I assume that this amp is pretty noisy when no guitar is plugged in?
All over there are things that raise my hair. Even for 150$ it’s a crime to sell this thing. IMHO looking at it makes me wonder whether it is safe to use. Be very careful!
I recommend stripping the entire amp and start over, using one of the schematics in the download area. The parts used are very poor quality, or mediocre at best. I would even get rid of the filter caps too, because by judging the rest of the job that’s been done I seriously doubt they were treated well.
OTOH, good enough platform to build yourself a decent amp.
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Re: Pentode (EF86) pre-amps and combo's?

Post by dwetherby »

Bieworm wrote:
Mon 04/29/24 3:22 am
The resistor on the input is the 1M grid leak resistor from the 1st amplification stage. It is really necessary, and more often than not right there where yours is.
I do see some odd stuff in your amp, apart from the poor soldering job all over.
The input jack is a switching type, but it’s not grounded at the switching. I assume that this amp is pretty noisy when no guitar is plugged in?

All over there are things that raise my hair. Even for 150$ it’s a crime to sell this thing. IMHO looking at it makes me wonder whether it is safe to use. Be very careful!
I recommend stripping the entire amp and start over, using one of the schematics in the download area. The parts used are very poor quality, or mediocre at best. I would even get rid of the filter caps too, because by judging the rest of the job that’s been done I seriously doubt they were treated well.
OTOH, good enough platform to build yourself a decent amp.

Hey man I really appreciate your input!! Can you tell me what looks so dangerous so I’m aware when I rebuild. Also do you recommend a brand of caps and resistors??? I think when k rebuild it I’m going to incorporate the 3rd ax7
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Re: Pentode (EF86) pre-amps and combo's?

Post by Daviedawg »

Don't use it without looking at the grounding of the faceplate to be sure it has good continuity to earth. See my earlier message.

Dd
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Re: Pentode (EF86) pre-amps and combo's?

Post by Bieworm »

For starters: avoid inline connections. Every connection should have a solid terminal such as solder terminals or turrets/lugs. I don’t like the resistor between a wire and a terminal route. Also try to avoid too much naked wire. This can easily be accomplished with heat shrink.
Cap brands are less important, but some of yours look like caps used in guitar electronics. When you don’t know what to use, then I’d recommend the Mallory 150’s for coupling caps, tone caps and cathode caps. Some values are logically requiring polarised caps. There are plenty of good brands for this. Filter caps preferably F&T. Resistors are 1W or higher in my amps.
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Re: Pentode (EF86) pre-amps and combo's?

Post by AussieTim »

Hi Guys
Anyone tried these tubes, thumbs up or down?
https://brimaruk.com
Tim
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Re: Pentode (EF86) pre-amps and combo's?

Post by JMPGuitars »

AussieTim wrote:
Fri 05/10/24 10:10 am
Hi Guys
Anyone tried these tubes, thumbs up or down?
https://brimaruk.com
Tim
Thumbs down. They're not making anything. They're rebadging tubes from other manufacturers and charging extra. Very similar to TAD, but TAD has a better track record. Their "why buy from us" section reads to me more like why not to: https://brimaruk.com/valves/brimar-tube-range/ - scroll down to find it there.
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Re: Pentode (EF86) pre-amps and combo's?

Post by AussieTim »

Thanks Josh, it is a real puzzle buying valves, some are comparatively priced here some are ridiculously overpriced, some people bang on about NOS others say the new manufacturing is actually better than the old stuff and NOS is a waste of money
Most places here in Australia just sell JJ's and Electro Harmonix or a limited range of other tubes
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Re: Pentode (EF86) pre-amps and combo's?

Post by JMPGuitars »

There's no shortage of opinions on tubes. 😉

EH is fine, JJ is not.

Thanks,
Josh
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