Tubelab SPP with EL86/6CW5
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Mark Forums Read

 Tubelab Discussion and support of Tubelab products, prototypes and experiments

 15th April 2019, 01:57 PM #1 OldHector   diyAudio Member   Join Date: Jun 2018 Location: Täby, Sweden Tubelab SPP with EL86/6CW5 I have just finished my 2nd SPP, this one using the EL86 variant. After one teething problem - due to the difficulty reading the colours on those tiny 1/4 watt resistors (I am going to start to use 0.6 watt to make it easier) - I had a living, breathing amp, producing sweet music. The sound was comparable to my 6P14P (Russian EL84) one. However after 20 minutes or so I noticed the tubes were starting to take on a orange-ish hue, the dreaded red plate. Checking voltages, I see (with some variation between the tubes) V(k) - 17v V(a) - 210v V(g1) - 0.02v V(g2) - 211v Voltage across the screen resistor (100 ohms) is 0.3v, so I(g2) = 3ma Voltage across the cathode bias resistor (270 ohms) is 17v, so I(k) = 63ma Plate voltage is (210 - 17) = 193v. Therefore dissipation is 193 * (63 - 3), which is 11.6W. Assuming my maths is OK, and my rookie knowledge is not too lacking, then I suppose that dissipation figure is not so high, but I have read that cathode current should be restricted to 50ma. Should I contemplate a 390 ohm bias resistor? Out of interest - why is the bias resistor 5W? Is there a spike in voltage or current at some point? And also, what is the benefit of having seperate resistors? Lots of PP designs using a common resistor, and a lot of the tube data assumes a single resistor in a PP setup. I have two pairs of tubes, one is Philips NOS and the other is HQ, which looks like an old brand but I had not heard of before I bought them. The HQ tubes are not as well balanced as the Philips ones (19.6 and 15.6 V(k)), and I suppose that is down to them not being a quality brand. Does anyone ever fit a balancing trimmer from each of the bias resistors with the wiper going to ground?
Tubelab_com
diyAudio Member

Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
 Out of interest - why is the bias resistor 5W?
In your example with the mismatched tubes you see 1.4 watts dissipated in the cathode resistor. Common engineering practice calls for a 2X safety factor on all resistors with respect to power rating, so technically a 3 watt resistor could suffice. This board is also used with 6BQ5 / EL84 tubes which need a 5 watt resistor, so the board must accommodate 5 watt resistors. The common 5 watt white ceramic and sand resistors are cheaper than a 3 watt resistor, so the choice is easy......put a 3 watt resistor there if you want.

Quote:
 And also, what is the benefit of having seperate resistors? Lots of PP designs using a common resistor
one resistor and one cap for a push pull amp WAS common. That was done in the days when tubes were cheap, and caps were expensive, but it is a compromise. Much of the AC currents cancel, so a smaller value cap can be used. You won't find too many modern HiFi designs with a common resistor.

Quote:
 Should I contemplate a 390 ohm bias resistor?
If the tubes are showing plate color you should reduce the current. Increasing the cathode resistor is the easiest way to do this. My guess is that a 330 ohm may be OK, but you might try both 330 and 390 ohm to find the best compromise between sound quality and tube life.

There are lots of different flavors of "6CW5's, and quite a bit of variability between them. I have used mostly GE 6CW5's in my SPP amps and the 270 ohm works OK. I tested some UL84's that only state "made in France" and they seem to want less current in a SPP. They were actually used in a guitar amp where they worked well with 340 volts on the plates and 170 on the screen grids. I was getting over 20 watts from them without any glow at all.
__________________
Tubelab, I blow stuff up so that you don't have to.

 18th April 2019, 10:09 PM #3 OldHector   diyAudio Member   Join Date: Jun 2018 Location: Täby, Sweden Hi George, thanks for the explanations, which make perfect sense now I can see the design rationale. I’ll experiment with the cathode bias resistor. Hoping i’ll be lucky enough to have some candidates knocking about, otherwise it will be a long wait until the spares shop opens again on Tuesday ;-/
 26th April 2019, 09:05 AM #4 OldHector   diyAudio Member   Join Date: Jun 2018 Location: Täby, Sweden I have suspended this build because I don't think it is viable. I have a power supply that provides a B+ of 211v, and I have OPTs that have ultralinear taps. The goal was to limit screen = plate at 200v, and run the amp in ultralinear config. However the tubes soon start to glow red in that setup, and now I think it is the screen that is overheating, and that the tube is partially running away and partially overheating due to a design limitation (screen voltage) being overstepped. So I think there are two options from here if I retain the EL86. 1) Modify the power supply to look like The Red Light District supply, with a dedicated regulated screen supply. Then configure the setup to be pentode. 2) Reduce B+ to max 187v, which would also impact the driver and phase splitter supplies, so might necessitate some re-jigging there. The alternative is to just put EL84's in the board, and source another power supply transformer with higher volts. Another solution would be to find a tube that has an operating point of Vg2 = Vp = 200V (I.e a special EL86) - does anyone know a tube that has a normal specification like that?
 26th April 2019, 12:49 PM #5 Tubelab_com   diyAudio Member     Join Date: Jul 2005 Location: West Virginia panhandle A true GE 6CW5 will run at 200 volts in fixed bias. The spec sheet actually shows 250 volts plate, and 200 volts screen with a 3000 ohm load for 25 watts power output in fixed bias push pull class AB1. They show an idle current of 91 mA for two tubes at 250 volts which is 11.25 watts per tube in their data sheet. These tubes eat that without issue, but not all tubes are made that tough. I was running GE 6CW5's in my board at about 230 volts plate and screen in cathode bias mode. That puts about 210 volts across the tube. All my tubes were pulled out of dead HP audio oscillators, so they were well used when I got them. __________________ Tubelab, I blow stuff up so that you don't have to.
 29th April 2019, 09:24 PM #6 Pooch   diyAudio Member   Join Date: Nov 2010 Oldhector, what output transformers do you have? I built a SPP EL86 with Sansui 1000 tube receiver outputs.
 2nd May 2019, 08:57 PM #7 OldHector   diyAudio Member   Join Date: Jun 2018 Location: Täby, Sweden Just had a look at the Schematic for the sansui, and they are not ultralinear. My ones were from toroidal and were 3.5k impedance, with 43 % ultralinear taps. I was trying to take advantage of the UL taps, but don’t think it is possible with a standard EL86. What screen voltage did you have? My plan is to mod the power supply and have a dedicated screen supply. Cheers, Richard
 2nd May 2019, 09:38 PM #8 Pooch   diyAudio Member   Join Date: Nov 2010 I can't remember, but I built it exactly as per George's instructions using an Antec IT200 PT. It works well, but I got an email from a member who said a IT220 worked even better?
 7th May 2019, 10:02 AM #9 OldHector   diyAudio Member   Join Date: Jun 2018 Location: Täby, Sweden The Antec IT200 should give you about 189v B+, if my reading of the specs is correct, which should just about be perfect for the max rating of the screen, i.e. Vg2 = Va = 170v. (This is assuming the pessimistic reading of the tube data sheets). That could be an option for me. However, I am quite keen to investigate power supplies a bit more, and look at regulated screen supplies, since I think that opens up some avenues for some of my other ex-TV tubes.
 16th June 2019, 06:08 PM #10 imnewbie   diyAudio Member   Join Date: Jun 2010 What type of choke can be used for EL86 amp?

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Room Acoustics & Mods     Music     diyAudio.com Articles     Car Audio     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post mctavish Tubes / Valves 43 23rd April 2019 02:03 PM rifraf50 Instruments and Amps 16 6th June 2016 09:54 PM andyjevans Tubes / Valves 7 15th April 2013 03:45 PM hartze11 Tubelab 1 27th May 2010 03:16 AM genome Tubes / Valves 0 17th October 2005 02:49 PM

 New To Site? Need Help?

All times are GMT. The time now is 06:36 PM.

 Home - Contact Us - Advertise - Rules - diyAudio Store - Sponsors - Privacy Statement - Terms of Service - Top - Opt-out policy

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.