Problem with this fizzy sound

[diagrammatiks]

you could but if you already snipped the caps in the power section there's really no reason to but them back in.

Plus since they are ceramic the footprint for them are really small...you'd have to do an upright bend to fit a sprague or tad in there.

If you really wanted to replace them, I'd suggest a wima mkp2.

[sickman82]

Sorry, I was a bit unclear with what I meant.. I am going to replace the ceramics with either new ceramics or WIMA, as you suggest because I soldered the leads back together on the original ceramics. I just wondered whether there was any point in replacing the film caps that are in there as well while I am already having to replace the ceramics. I thought it might be worthwhile getting some nice, reliable European made caps for the amp.

[diagrammatiks]

I really don't think that'll be necessary. Vox already went above and beyond with their power supply bypassing. It is unlikely that you will see that big of a difference.

[Steve Conner]

Yeah, I agree. Classic amps used no bypassing across power supply capacitors, and whatever coupling caps they could get cheaply.

By the way, the 220k plate resistor and 1.5k cathode resistor are the values used in the AC30 Top Boost preamp.

Circuit Design of the Vox AC30 Top Boost Preamp

Except doesn't the AC30 share that 1.5k cathode resistor between two triodes? So that works out as 3k per triode, a better match to the 220k plate load.

The above article is also the only place I ever heard the term "gamma network".

[Chuck H]

I've speculated here before that tiny interactions in the power supply due to ESR (which AFAIK increases with requency) may form negative and positive feedback loops that affect the overall tone of an amp. For better or worse depending on several facors. I can barely hear a difference in small value changes in power supply caps of the same age and brand. But I can usually hear a significant difference between caps with higher ESR and those with less. There was a long discussion at TAG concearning the best caps for the Express builds they do. Fairly non scientific but in that particular amp design the wrong caps make the amps harsh and the right ones mellow it out. It turns out that the right caps were some boutique product that were supposedly made to behave like caps of yesteryear. I'll interpret that to mean higher ESR. So there you go... My $.02. I guess that means that if the amp sounds better with the ceramic caps bypassing the power supply filters then go ahead and put them in. Otherwise leave them out. Any benefits or detriments due to the ESR of the filters would be greatly reduced by the bypass caps.

[diagrammatiks]

the top boost also runs that first stage at a b+ of 375 so the bias point is right in the middle.

also ya 2 triodes share the cathode resistor so it's effectively 1.5k.

[Steve Conner]

Surely 275V, not 375?

[diagrammatiks]

yes 275. richard's graph is wrong on pentodepress. whoops.

[sickman82]

Check this out...

preamp
R9 220K 100K/0.5W Carbon comp
C21 1.0uF/50V 10uF/50V Fine gold
R26 220K 100K/0.5W Carbon comp
C7 2.2uF/50V 10uF/50V Fine gold
C29 250pF 250pF/500V Silver mica
C31 0.022uF/400V 0.022uF/630V Mallory
C32 0.022uF/400V 0.022uF/630V Mallory
R25 1M 220K/0.6W metal film
next C27 47pF 100pF/500V Silver mica
C22 0.022uF/400V 0.022uF/630V Mallory
next C25 0.1uF/400V 0.1uF/630V Mallory
C23 10uF/35V 10uF/50V Fine gold
C22 0.022uF/400V 0.022uF/630V Mallory

phase inverter
R11 100K 100K/0.5W Carbon comp
C10 0.1uF/400V 0.1uF/630V Mallory
C16 0.1uF/400V 0.1uF/630V Mallory
C15 0.1uF/400V remove & link
C30 0.1uF/400V remove & link
C26 0.1uF/400V 0.1uF/630V Mallory

power amp
R4 120R 130R/5W Wirewound
R7 470/1W 110R/3W metal oxide
R8 470/1W 110R/3W metal oxide

power supply
R1 1K/2W 470K/2W metal film
R2 220/5W 470R/3W metal oxide
R3 220/5W 470R/3W metal oxide
R17 18K/1W 12k/2W metal film
couldn't be happier

These are the mods to the Night Train from a forum member over at Metroamp. He shows original value, then the value/type he has used in its place. Notice how he has reduced R9 and R26. What would you say he has achieved with these mods? I have asked him as well, but would like to get your input.

[diagrammatiks]

just the same sort of things we've been talking about.

dropping the resistor from 220k to 100k gives you more head room.

[sickman82]

I understand that, and a few others.. But why remove and bridge C30 and C15 for example? Can you see any benefit to using carbon comp resistors? What I'm getting at is whether anybody thinks there is anything here we can use to help solve our problem..

[Chuck H]

These are mods that worked for one guy in one place. And as such he is likely to be biased (pun intended) to think the amp is now the bees knees. These mods DO put the amp into better operating parameters. But there is a lot of changes there. The amp will be a totally different amp. If you think you would like what this guy likes based on what MUST be a very biased opinion then do it. Otherwise we can get back to trying to fix the trailing crackle. If R13 actually measures 4.7k then reduce it to 2.2k. and reduce R16 to 4.7k. Change the 220k plate loads (R9 and R25) to 150k. Change the preamp bias resistors (R23 and R10) from 1.5k to 1k.

These mods will put the preamp bias and voltage in a more stable operating range and probably won't change the overall character of the amp that much. Hopefully it wil get rid of the crackley decay. But someone needs to do the mods instead of talking about them to see. I don't have one of these amps or I would do it.

Carbon comp resistors have some "mojo" but it's really tiny. In high gain preamps (like yours) it's a bigger detriment because of added HISSSSS. I would use metal film resistors for plate loads in any high gain preamp. I did a project recently where carbon comp resistors were spec'd. I did however use metal film for plate loads in the preamp and any high series resistances in the preamp. The amps were quiet and well behaved. But carbon comp plate loads or high series resistances in preamps are a sure way to make an amp noisy as hell. JM2C

[sickman82]

Yeah, I had a feeling that using carbon comp resistors would be a bad idea. Even in the pedals that I build, it is usually a good idea to steer clear of them unless you want to make a vintage correct reproduction. It is just too noisy. This amp has a load of gain on tap, it gets pretty noisy as it is.. with carbon comp resistors on top of that? Not something I would do.

Is there anything in the circuit that could have an effect on how R13 measures with a DMM? I measured it in circuit.

I'm waiting for my wage before I can make these mods. Totally blown out after Christmas etc. But I definitely plan on doing the suggested mods Chuck, don't you worry.

[Chuck H]

Is there anything in the circuit that could have an effect on how R13 measures with a DMM? I measured it in circuit.

I don't see anything that should affect the reading. Sometimes voltages present will skew a resistance reading but since 4.7k is a common value and didn't drift while measuring I'd bet it's correct. Probably just a post schematic design change.

[Silvio55]

Well I made the order of the components thanks to a guy at a TV repair shop, while most of the mods listed at the metro amp forum are just changing the components brand or type but not the value, thereīs something I donīt understand....

Check this out...

preamp
R9 220K 100K/0.5W Carbon comp
C21 1.0uF/50V 10uF/50V Fine gold
R26 220K 100K/0.5W Carbon comp
C7 2.2uF/50V 10uF/50V Fine gold
C29 250pF 250pF/500V Silver mica
C31 0.022uF/400V 0.022uF/630V Mallory
C32 0.022uF/400V 0.022uF/630V Mallory
R25 1M 220K/0.6W metal film

R25 is a grid leak resistor right? Whatīs the porpouse of dropping down the value from 1M to 220k?