Bench Phono

Will · Post by **Will** » Wed Nov 19, 2008 12:13 am

Richard
my pm before your post covered most things but the cap size I use is 2.2uf in the pre and phono amp after trying upper and lower values, but thats me. It can be in the upper or lower primary leg, lower leg for me.
Don't get carried away, this is not a interstage it is a bridging transformer and is o.k. on the output of the above amps only if the output is lower than 15k imp.

richardcooper2k · Post by **richardcooper2k** » Wed Nov 19, 2008 8:37 am

ok i'll try and work out what my output impedance is - i think morgan jones tells you how

and i'll get hold of a couple of the altecs to have a play with

do they fit into a certain type of valve base ?

i was trying to work out wether the cap value was different depending which side of the winding it goes

but i don't need to know everything before i can play

richardcooper2k · Post by **richardcooper2k** » Wed Nov 19, 2008 11:49 pm

is the 15335 any use ?, no 15335a on ebay at the moment that will ship outside usa

was also wondering if any of the cinemag 15k:15k were as good ? i like their step ups and the price seemed reasonable for those

Will · Post by **Will** » Thu Nov 20, 2008 11:54 am

Ask Cinemag they will know the 15335A, Octal pins, the 15335 is maybe the earlier model with less screening.
I have a pdf of the 15335a Specs can't link it here but PM me your email.

richardcooper2k · Post by **richardcooper2k** » Sat Nov 22, 2008 12:50 am

hmm. typical anode resistance for 6072 is 25k and the anode load resistor on the last stage of my phono is 100k. paralelling these give an output resistance of 20k

if i've got that right my output resistance is higher than the max suggested of 15k

so do i need to look at transformers with higher resistance windings ?

richardcooper2k · Post by **richardcooper2k** » Sat Nov 22, 2008 10:33 am

hi nick. i have a vague recollection at owston of will saying something about the large value of the anode load resistors in my phono and you saying it wouldn't matter as whatever the problem was (i don't remeber as i didn't understand) it would be effectivly governed by something else. can you remeber, was that to do with output impeadance ?

Will · Post by **Will** » Sat Nov 22, 2008 12:03 pm

I think you will be o.k. (RL+RA)/(MU+1)=output Z so this gives 4-5k if the maths is correct Nick, Nick??
You could also use a CCS as a 300H anode choke would be needed to compete with it, 2 hammond 150H, depends on space.
The 15335a are about 33H on the primary so a 2.2-4uf cap.

richardcooper2k · Post by **richardcooper2k** » Sat Nov 22, 2008 10:59 pm

now there's a coincidence as i was planning on trying an anode choke there anyhow and ordered 2 off 150H hammonds last night. sounds like i might need to get a couple more.....

i would like to try to understand as well though. why do i need 300H ? nothing much on annode chokes in morgan jones, where can i read about them ?

morgan jones has equations for the cut off frequencys of CR anf LR filters but not CL filters. what is the formula used to calculate the 2.2uF coupling cap in this case ?

Will · Post by **Will** » Sun Nov 23, 2008 12:31 am

The anode choke needs to be large enough to near a 1meg load which is twice the technical calculated figure at say 10hz, a CCS I think is infinate so the larger the choke the better as they say. AnodeRes/30 is near 5Hz figure.
Cap calc'r.
http://ourworld.compuserve.com/homepage ... en/XLC.htm

Will · Post by **Will** » Sun Nov 23, 2008 1:31 am

Richard read this by Garry Pimm on Hammond chokes
http://db.audioasylum.com/cgi/m.mpl?for ... late+choke

Post by **Nick** » Sun Nov 23, 2008 10:16 am

If I have the numbers right, output impedance for a common cathode amplifier is

RL || ra

where || in parallel with. If the cathode is unbipassed, the anode resistance is increased by

ra' = ra + RK( 1 + µ )

Richard, the choke size matters, as the anode resistance of the valve forms a RL high pass filter with the choke, so the values Will is giving are shortcuts to a sensible -3bd point for the circuit, but you can use the normal LR equation and ra to work out exactly what you get.

Its possibly worth checking on the plate curves for the valve what actual ra you are getting at the op point of choice, as the spec's tend to give the minimum value.

richardcooper2k · Post by **richardcooper2k** » Tue Nov 25, 2008 1:07 am

got a reply the next day from david geren at cinemag:

Hi Richard;

The Altec 15335A was 15K:15K line coupling transformer. The CMLI-15/15B does the same job, but is a better transformer. The CMLI-15/15B sells for $45.02, but not in an octal can. We are really busy at this time. Doing one in an octal can will take a lot of time and costs enough more that you might as well buy one on EBay.

Cheers,
David

which i think is no more expensive than the altec are going for on ebay anyway. so i think i'll order some. anyone else want to share postage ?

richardcooper2k · Post by **richardcooper2k** » Tue Nov 25, 2008 1:27 am

can someone explain what is happening when the anode choke is competing with the primary of the transformer ? is this what is going to effect my freq resp if my choke isn't big enough ?

i can't find curves for 6072 in the web, can anyone point me in the right direction (to work out ra) ?

what will happen if my output imp is higher than 15k ? (freq resp again ?)

thanks

Post by **Nick** » Tue Nov 25, 2008 7:55 am

If you have a anode choke and a paralel feed transformer, then the two inductors will be in parallel. Also the secondary load will be reflected back and be in parallel with the valve Ra.

http://www.mif.pg.gda.pl/homepages/fran ... 6/6072.pdf

richardcooper2k · Post by **richardcooper2k** » Tue Nov 25, 2008 9:03 pm

so if the choke and transformer primary are in parallel, the lower the L of the primary, the less effect the anode choke will have ? so if the trans primary is 33H, even with a 300H choke, effectivly it will only be 30H ? will i gain anything from 30H of anode chokeage ?

using the 6072 data in the link (thanks nick) i got ra at about 26k (vg =1.7v, HT=260V, RL=260V) . so in parallel with 100k RL gives 21k output resistance which is a bit high. what will be the effect of this and how can i get round it (different transformer ?)

but......if the secondary load is reflected back in parallel with Ra, then effective Ra and therefore output resistance will be decreased so it might be all ok afterall ?

audio-talk

Bench Phono

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