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Vertical 6EM7 Portable Amp
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Author:  Suncalc [ 03 Sep 2015, 14:55 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Yes. I do this regularly.

The thing about SETs is that their current load doesn't change very much between zero and full output. For this project you're looking at about 86mA total load at 260VDC. This equates to a just about a 3k load dissipating about 22.4W. I would use Vishay Dale 50W housed resistor (like this one http://www.alliedelec.com/vishay-dale-rh0503k000fe02/70201501/) as a test load, because they derate to about half 40% of their rating power in open air (in this case 20w) so not much heat sink is required.

Keep us informed on your progress and congratulations on your first vacuum tube project. :beerchug:

Author:  mwhouston [ 03 Sep 2015, 19:49 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

Geek, I do the same. Roughly workout the current drain at say 260V which could be expected then add resistors as a load to roughly check PS. It's rough but gives you a starting point. When tubes are connected I start with far too much limiting resistance on the PS and keep backing it off (usually using jumper leads) until the desired HT is found.

Author:  wdecho [ 07 Sep 2015, 07:29 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

The PS transformer I have has a center channel wire for the filament voltage. Should I ground this wire and would this be a problem if grounded or is it necessary to ground? I heard somewhere that if grounded it will reduce hum.

Being my first tube build I have found that a lot more holes have to be drilled vs a SS amp. A lot more cool looking than a plain box.

Author:  Suncalc [ 07 Sep 2015, 10:52 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

wdecho wrote:
The PS transformer I have has a center channel wire for the filament voltage. Should I ground this wire and would this be a problem if grounded or is it necessary to ground?
If you have a separate heater supply winding with a center tap for your rectifier tube, it should not be grounded. It will not reduce hum and if you have a filamentary cathode on the rectifier, it will short out the high voltage supply through the transformer and destroy it. In general, independent rectifier filament supplies should be left to float (or follow the B+ voltage when using filamentary cathode types).

Author:  wdecho [ 07 Sep 2015, 18:26 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I finished my 6EM7 today and I am truly impressed. This is an amp I could truly live with without any regrets. Lots of soundstage and not lacking in any part of the audio range. Absolutely quite with my 103 db horns and plenty of power. It actually had plenty of power with some 87 db test speakers. I will post more listening impressions later. I found some Sylvania 6EM7 with very little use and have also bought some NOS black plate coin base on Ebay for $8 apiece to compare at a later date. Total cost with a conventional case was around $250. I am grateful to have a SE SET Class A amp for this low cost. Thanks Matt for sharing this amp.

Author:  mwhouston [ 07 Sep 2015, 18:32 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I sold and made a couple of really great 6EM7s. Regret both sales.

Author:  wdecho [ 08 Sep 2015, 06:16 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

mwhouston wrote:
I sold and made a couple of really great 6EM7s. Regret both sales.


I read about your builds before building my amp. I was concerned about 2 watts being enough but that was no problem with my 103 db horns. I was also concerned about hum with such efficient speakers but it is inaudible with my ear next to the horn. A large wide soundstage with pinpoint centering of the vocals. Decent bass with a nice midrange and treble. I have built 3 Firstwatt amplifers and the sound is a lot like the F6 for a lot less money. There are some differences but nothing that really stand out. This is an amplifier I could be totally satisfied with in my system. The only problem I had was no voltage on the output tubes to start with because of wrong wiring on my part. After 30 minutes I found and fixed the problem. On start up if there is no sparks and no magic smoke I consider the battle close to being won. About as easy a build as one will find for point to point with few parts and only 3 tubes. The hardest part was drilling holes and layout for the chassis. It took around 3 or 4 hours to build the power supply on one day and about the same time to build the amp part the next. I have always thought the sound was very similar between Class A tube or SS and this has confirmed my thoughts. For a cheap build this is a very satisfying amplifier and most will find 2 watts of this amp being plenty.

Author:  wdecho [ 08 Sep 2015, 06:22 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I was only able to achieve 252 volts of the B+ instead of 260 volts called for. I was using a rather cheap volt meter and it could be off some so I proceeded on. It sounds perfectly fine as is and I could always take the rheostat out and lower the 390 ohm resistor but I really do not think this is necessary. Any thoughts someone?

Author:  mwhouston [ 08 Sep 2015, 07:54 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

I wouldn't worry about 8V in 260V. My very best amp, was Abstinence with the K&H opts and Sophie mesh plate rec tube. Unbelievable bass from cheap Chinees OPTs.

Author:  wdecho [ 22 Sep 2015, 05:40 ]
Post subject:  Re: Vertical 6EM7 Portable Amp

If I disconnect an input cable from one channel I still hear sound coming from both speakers. I take this as crosstalk between channels or channel separation problem. I thought it might be my output transformers talking to each other so I placed a shield around one of them which did not change anything. I have not went back inside the amp to take a look yet because as far as the sound it sounds great with a big large soundstage better than any amp I have heard and I do have different instruments on one speaker or the other so I do have some separation. All the SS amps I have built, if I disconnect one channel I kill the sound from that speaker. My pot is not the best so I am thinking it may be the problem and I have used it before. As I have said there is not a problem with the sound I just notice this by accident, I just wonder if it is something I should diagnose and try and better.

I built the PS as in the schematic with the 3 chokes.

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