R
richedie
New member
I had a conversation with one of the guys at Eurotubes and they said the tech's at Bogner are wrong concerning the bias of EL34s in the 20th Shiva. Basically, he went on to say how Bogner makes some of the best amps but they don't know much about tubes and plate voltage, etc. He went on to explain and it makes complete sense. He said he has already retubed some of the 20th Anniversary models, some with 6L6s and some with EL34s and all biased hotter than Bogner recommends because he says they are wrong. He recommends 30-35ma for EL34s. I am doing it!
He said:
Bogner says that the Shiva can only use the =C= tubes because the plate voltage on the Shiva is so high and it runs right at 500 to 515 plate volts. This is of course incorrect…. If you read their site further you will find that Bogner recommends running ONLY the JJ EL34’s in the Uberschall because the plate voltage is so high and it runs right at 520 to 525…
Moral of the story? Bogner builds great sounding amps but they know nothing about tubes. The JJ EL34’s are capable of 700 plate volts where the JJ E34L’s are good to 750 and the KT77’s to 800. The JJ 6L6GC’s are good for 600 and we constantly use them in old Ampeg amps that run 560 to 580 where a =C= tube will fry at these voltages.
Bogner will tell you that you cannot use 6L6’s in one of their EL34 Shiva’s and that you cannot run EL34’s in one of their 6L6 amps. This of course is not true. Bogner makes very nice sounding amps, in fact the Shiva is my second favorite right behind the old Caveman but they don’t know much about tubes.
Anyway, in an absolutely perfect world the tranny used for EL34’s which have an impedance factor of about 15K would match up a little better with a tranny made for them than 6L6GC’s which have an impedance factor of about 22K would, but you won’t hurt anything running 6L6’s with a tranny would for EL34’s. KT77’s and KT66’s have an impedance factor of about 23K so they actually match up with the 6L6 tranny perfectly. KT88’s have an impedance factor of about 12K so they actually match up with the EL34 tranny fairly close.
The bottom line is if this mismatch were a problem then all the mass quantities of amps like Mesa, Diezil, Peavey, Laney, Orange, Traynor and VHT to name a few would be going up in smoke because they can all use multiple tube types and they do not use different trannys to do this.
We have been using 6L6GC’s in EL34 Shiva’s and EL34’s, E34L’s and KT77’s in 6L6 Shiva’s for many years now and never once encountered any issues. The only consideration is that EL34’s draw only about half as much natural plate current as 6L6’s do so if you want to use 6L6’s in an EL34 Shiva then you need to stay with cooler grades so you can get the bias to between 32 to 38mA. The answer to you next question is no, cooler grades don’t sound any different or breakup any later than hotter grades, it’s where you set the bias that makes the most difference as to when distortion sets in.
Your amp is an adjustable bias amp (not floating bias!). The reason Bogner tells you to bias EL34's at 29ma is due to the plate voltage being over 500. Any adjustable bias amp needs to have the bias set when you change power tubes and we highly recommend learning to do this so that you are not at the mercy of a tech who will charge an average of about 50.00 to adjust the bias
Yes, the bias does change a bit as you play. It doesn't stay at a higher setting, it going up and down a bit as you play. These amps have around 500-515pv so let's review our bias formula:
The formula for calculating where you should set the bias is:
Max dissipation of the tube divided by the plate voltage and then multiplied by anywhere between 65% and 80%.
Lets use 25 to represent the max dissipation of EL34's/E34L's/KT77's. We'll use 500 to represent the average plate voltage.
25 divided by 500 = .0500 or 50 milliamps X 0.65 = .0325 or 33mA
25 divided by 500 = .0500 or 50 milliamps X 0.70 = .035 or 35 milliamps
25 divided by 500 = .0500 or 50 milliamps X 0.80 = .040 or 40 milliamps
The 50mA is the number you would set the bias too if you wanted to run the tubes at 100% of there capability at idle. If you did this, once you started playing and the tubes where producing power the bias would jump above 100% and burn up. So we usually set the bias to 70% of maximum dissipation (or around there depending on how you play and what your looking for) so that when the tubes are working hard we can be fairly certain they are not being pushed past there limits.
We have retubed many, many Bogner Shivas using this same formula. We also have tons of customers who have retubed their Bogners including some 20th Anniversary models. using this same formula. We don't have trouble with tubes burning up! To be safe however, set the bias from 30-35mA. You won't have any troubles by doing this!
Again, Bogner builds great sounding amps but they know nothing about tubes. The JJ EL34’s are capable of 700 plate volts where the JJ E34L’s are good to 750 and the KT77’s to 800. The JJ 6L6GC’s are good for 600 and we constantly use them in old Ampeg amps that run 560 to 580 where a =C= tube will fry at these voltages
He said:
Bogner says that the Shiva can only use the =C= tubes because the plate voltage on the Shiva is so high and it runs right at 500 to 515 plate volts. This is of course incorrect…. If you read their site further you will find that Bogner recommends running ONLY the JJ EL34’s in the Uberschall because the plate voltage is so high and it runs right at 520 to 525…
Moral of the story? Bogner builds great sounding amps but they know nothing about tubes. The JJ EL34’s are capable of 700 plate volts where the JJ E34L’s are good to 750 and the KT77’s to 800. The JJ 6L6GC’s are good for 600 and we constantly use them in old Ampeg amps that run 560 to 580 where a =C= tube will fry at these voltages.
Bogner will tell you that you cannot use 6L6’s in one of their EL34 Shiva’s and that you cannot run EL34’s in one of their 6L6 amps. This of course is not true. Bogner makes very nice sounding amps, in fact the Shiva is my second favorite right behind the old Caveman but they don’t know much about tubes.
Anyway, in an absolutely perfect world the tranny used for EL34’s which have an impedance factor of about 15K would match up a little better with a tranny made for them than 6L6GC’s which have an impedance factor of about 22K would, but you won’t hurt anything running 6L6’s with a tranny would for EL34’s. KT77’s and KT66’s have an impedance factor of about 23K so they actually match up with the 6L6 tranny perfectly. KT88’s have an impedance factor of about 12K so they actually match up with the EL34 tranny fairly close.
The bottom line is if this mismatch were a problem then all the mass quantities of amps like Mesa, Diezil, Peavey, Laney, Orange, Traynor and VHT to name a few would be going up in smoke because they can all use multiple tube types and they do not use different trannys to do this.
We have been using 6L6GC’s in EL34 Shiva’s and EL34’s, E34L’s and KT77’s in 6L6 Shiva’s for many years now and never once encountered any issues. The only consideration is that EL34’s draw only about half as much natural plate current as 6L6’s do so if you want to use 6L6’s in an EL34 Shiva then you need to stay with cooler grades so you can get the bias to between 32 to 38mA. The answer to you next question is no, cooler grades don’t sound any different or breakup any later than hotter grades, it’s where you set the bias that makes the most difference as to when distortion sets in.
Your amp is an adjustable bias amp (not floating bias!). The reason Bogner tells you to bias EL34's at 29ma is due to the plate voltage being over 500. Any adjustable bias amp needs to have the bias set when you change power tubes and we highly recommend learning to do this so that you are not at the mercy of a tech who will charge an average of about 50.00 to adjust the bias
Yes, the bias does change a bit as you play. It doesn't stay at a higher setting, it going up and down a bit as you play. These amps have around 500-515pv so let's review our bias formula:
The formula for calculating where you should set the bias is:
Max dissipation of the tube divided by the plate voltage and then multiplied by anywhere between 65% and 80%.
Lets use 25 to represent the max dissipation of EL34's/E34L's/KT77's. We'll use 500 to represent the average plate voltage.
25 divided by 500 = .0500 or 50 milliamps X 0.65 = .0325 or 33mA
25 divided by 500 = .0500 or 50 milliamps X 0.70 = .035 or 35 milliamps
25 divided by 500 = .0500 or 50 milliamps X 0.80 = .040 or 40 milliamps
The 50mA is the number you would set the bias too if you wanted to run the tubes at 100% of there capability at idle. If you did this, once you started playing and the tubes where producing power the bias would jump above 100% and burn up. So we usually set the bias to 70% of maximum dissipation (or around there depending on how you play and what your looking for) so that when the tubes are working hard we can be fairly certain they are not being pushed past there limits.
We have retubed many, many Bogner Shivas using this same formula. We also have tons of customers who have retubed their Bogners including some 20th Anniversary models. using this same formula. We don't have trouble with tubes burning up! To be safe however, set the bias from 30-35mA. You won't have any troubles by doing this!
Again, Bogner builds great sounding amps but they know nothing about tubes. The JJ EL34’s are capable of 700 plate volts where the JJ E34L’s are good to 750 and the KT77’s to 800. The JJ 6L6GC’s are good for 600 and we constantly use them in old Ampeg amps that run 560 to 580 where a =C= tube will fry at these voltages
not sure how to recieve all this info.
That being said, I may go back and re-read your post as it is lenghty and make sure I did not miss something or mis-interpret anything the wrong way.. But trust me when I say those two fellows down at Bogner I know,......ARE NOT THE ONES DRIVING THE DUMB WAGON.....
I like to have an explaination with these things.