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In Reply to: EL509 - best bet for my 1K-p-p xformers? - got em for bass - have extended HF posted by freddyi on July 12, 2001 at 00:37:16:
Hi Freddy,the EL509 gives very nice 100W in the following configuration:
Up=450V
Us=150V
Ug1=-42V
Raa=2kohmsFor 1 kohm output transformer just put 4pcs. in paralell push-pull for 200W if the transformer can handle it. I think posssible to use only one pair of EL509 for this application, but with lower plate voltage such as 300V, and of course with same screen voltage rating.
Sajti
Follow Ups:
Hi Sajti -very much appreciate the 509 operating points - would you actively regulate the wcreen grid or will a separate stiff suppply work fine?
Talema could wind a nice toroid for low pirce - I'm not at all fond of toroids but don't know a source of affordable/good EI power transformers (the Hammon 278CX I got years ago run hot and windings buzz)- wonder if ~500-600VA of plate winding will do for a 200 watt AB1 amp?
the little 13lb M6 1K p-p output trasformers were built with a spec of 300 watts @40Hz - I think they have good primary L (quoted 80H) and may make a nice sounding amp even if they can't 200 watts @30hz
a few points - what size grid stoppers would you employ? - what size 509 grid to bias network resistor is safe?
driver issue - I'd like to keep it simple and nice sounding - depending upon the EL509 grid resitor it might be possible to just use a 12AT7 cathode-coupled driver preceded by a gain stage with no loop nfb - with CCS as "tail" the cathode-coupled inverter could have perhaps 10dB of output plate to driver plate feedback - it might simplify problems as less loop nfb would be required - what type driver would you pick? - some like a split-load. One Traynor bass amp used EL509 - probably 220K grid per pair(!) plus a 12AX7 driver with standard Fender-like plate loads of 82/100k - wiht a light load a 12AX7 using Rp=90K adn Ip 1.5mA/section could swing the outputs pretty well - unless grid current is creeping in fast.
Tahnks & Best Wishes!
Freddy
Hi there,the current of the screen grid of EL509 is very small. You can simply regulate it with zener string, or regulator valve (VR150). Idon't calculated the power consumption of this amplifier. But 600W looks OK for 200W amplifier.
The output transformer looks nice. I'm sure it can handle the power really nice.
I always use very small gridstopper resistor less than 470ohms, but I solder it directly to the tube holder with small leg.
I used 330kohms grid resistor for PL509, and it was OK. For high bias current I recommend to reduce to some 220kohms.
For driving try E88CC cathode coupled, with E88CC as driver. It gives very-very nice result for me. 12AT7 also possible, but I don't recommend 12AX7 because the '509 has very high input capacitance, because of the small distances inside the tube.The '509 gives nice triode, if You connect the screen to the plate, with some 47-100ohms series resistor. The application for this connection is the following:
Up=300V
Ug1=-75V
Ibias=100mA
Rpp=2kohms
Pout=38W
This application result almost pure class "A" application. The amplifier works in class "A" up to 20-22W. With double pairs You can get 70-75W...Sajti
Hi Sajti70 watts triode would be pretty nice withhout any loop nfb - my speakers run from ~95db to 103dB snesitivity -may sound "better" than pentode(?)- depending on application - I like 6BX7 in push-pull parallel with my Eico ST-70 iron and ~300 vak
using E88CC as cathode-coupled inverter - did you have a CCS "tail" or just offset plate load resistors? - may I ask some details of the driver with regard to plate loads, Rk, B+ and current?
thanks
Freddy
ps - for 150vdc screen supply - zener-referenced pass transitor OK? - think it would turn on slow with a cap across zener string
Hi Freddy,in my amplifier I used 12dB of loop NFB. Triode sounds much better than pentode, I think it's out of question. The problem could be only the power we can get.
In the driver I used E88CC both section parallel with 60V plate and 2mA bias, direct coupled to the inverter which runs with 120V plate and 4mA bias. I used offset plate resistor (I don't want to use CCS on the tail, because I try to avoid any SS parts from the amplifier).
The plate resistors for the inverter was 30komhs for the first and 36kohms for the second. Some adjustments may necessary for the second resistor. The driver stage used 300V HT regulated.Mcc
Hi Sajtithe operating points help - on the 1st parallel stage does 2mA translate to 1mA/section? - also assume 2/mA per section on inverter?
how "clean" is the triode-connected EL509? - only seen curves for screen-driven case - not triode - did you keep triode plots?is distortion and final Z-out low enough to run push-pull EL509 without any loop feedback and get good sound?
if it wasn't for the high driver demands in triode mode then local plate to plate feedback could be employed - not sure how much would be obtained in EL509 triode case.
take care,
Freddy
Hi,all the bias current I mean, for one section of the E88CC. The input stage 60V/2mA for each section results 62kohms plate resistor. The inverter 120V/4mA results 7.5kohms for the common tail.
The triode plots was measured by myself, I try to find the paper somewhere, and put it to the net. The result is low mu (approx 3) high gm, and low plate resistance (some 300-400ohms). This is very nice triode, the only problem You have to give very large input swing to get the output power. The output impedance very low thanks to the high bias current. Reduce the bias extend the tubes lifetime, but You need more urge to drive them.
If I want to build this amplifier again, I think I try to apply Williamson driver, but with E88CC in the second stage. This solution can result almost the double of the necessary voltage.
EL509 (300V/100mA) need 53Vrms for the full power. The driver I mentioned before can gives 65-70V before clip, but the Williamson with E88CC, and 150V/5mA can gives over 100Vrms.
My current project is PP triode amplifier with 6550s. For this design I planned double long tail stages for driver, with AC coupling, this also can gives over 100Vrms.Sajti
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