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Bravo Tube Head Amp Mod
Some of you guys might recognize or own this cheap head amp. Bought two of these many years ago from PE for ~$70. They can now be found all over the net for about the same price. This particular variant uses the 12au7 instead of the more popular 12ax7.
Definitely, IMO, not a highly resolving sound but very easy to listen to, powerful, and fun type of sound. My Aune and both boy's Schitt and cmoy headamps are more resolving. Anyhow, the tube in one of them turned white and it came to a halt. A new tube did not remedy the situation.
I've been on [https://rockgrotto.proboards.com/")] for years while rarely posting but none the less, enjoying the chatter.
Time to revisit an old mod thread specifically aimed at this cheap little headamp. https://rockgrotto.proboards.com/thread/5297
I'm really liking Mouser. They have a huge parts selection and parts are always well packed and arrive on time. Take note PE!
Time to mod. New irl530 to replace the irf630 that rolls off the highs, better lm317at voltage regulators, pair of 5k trim pots to dial in the bias which comes way off from the factory, and real Nichicon caps to replace the oem imitation Nichicon caps. There is also a crosstalk mod involving cutting a trace and solder a jumper. Still have the old original to compare it against.
Comments
I have one of these that I use when the recordings suck, seems to help bad recordings sound better. It does have a decent amount of power to help drive larger headphones as well.
If you say the mods are worthwhile I will likely follow suit. Please keep track of your parts list. 😀
As far as vendors go Mouser and Arrow are excellent. Mouser's search engine is the best.
I have one of these as well and will be following your mods with great interest. In stock form, it hasn't been able to displace my first gen Focusrite Scarlett in my remote working headphone setup.
Sehlin Sound Solutions
I'd really like a transistor version of this amp. You know, the case, heatsink, etc, and look like a combo of this Bravo and a Nagra Solid-state amp. I built a class-A headphone amp awhile back, but it's always had a hum problem.
Now that I have a nicely adapted set of cans that suit me, I'd really like a better amp for them.
InDIYana Event Website
If I was into headphones (or even owned a pair, I don't) I'd point-to-point DIY a single stage, SET, tube, headphone, amp in a beautiful case made out of polished aluminum parts with some hardwood pieces just for fun. There are several great tube designs out there and the prices for the transformers and tubes are really affordable. Nothing like the prices for a 35 wpc tube amp
A solder sucker and other stuff came in today but boy has an interview tomorrow so we wound up cleaning up his portfolio instead of modding the head amp.
The parts list is really simple. Maybe I can pull up the order from Mouser's website and post that, if this winds up being worthwhile.
I agree with Ken, it is easy on the ears but the irl530 mosfet isn't supposed to roll off the top end like the stock irf630 so this may not be the answer for someone wanting to tame hot recordings. As is, or as was, it had a fun and inviting sound, totally opposite of an analytical pick the fly $hit out of pepper sound. Hopefully these mods keep that inviting sound.
And I need to weight the oem capacitors vs the real Nichicon replacements. One post states that the 'imitation Nichicon' caps weighed noticeably less than their real replacements. I've seen pics of the caps on some of those cheap Chinese tube amps cut open where there's nothing but a tiny cap glued into a large cap body.
What are you looking for?
Magnifying glass.
Why do you need a magnifying glass? Is it time for bifocals?
No.
Do you need to borrow my bifocals?
No.
Do you need to borrow my reading glasses?
No. Quit bothering me.
Thirty minutes later after several failed attempts at soldering ...
What are you looking for?
Reading glasses ... and cue comments from the cheap seats.
I need something like that with a light and helping hands.
The watchmakers loupe comes in handy on a regular basis. I have a bench light from Lee Valley, it's nice and all and has 2x magnifier, but if you don't do electronics work often it might be a bit much. For the job it is perfect though with the ring light.
For helping hands check your local liquidation store. Locally there is several with tool sections, lots of junk tools but for some things there can be good deals. My favourite pair of clamps came from a liquidation store, as did my helping hands. I really only use the helping hands for tinning wires though, in all honesty I could get away with just any small clamp attached to a board to weigh it down. Use your bench vice if you have one.
Mods done. Amp still dead.
Jumpered the blue led and checked the red led. Still no dice. Not a lot more than a pair of 2n3906 and a handful of resistors remaining. Mouser again ...
Don't suppose there's an easy way to check the resistors without removing them?
Continuity checks work on most circuits as long ans there isnt another component in parallel. If you need me to take measurements off mine let me know.
Continuity, resistance, and voltage drop can all be done in-circuit.
That would be cool. Would any of the mods make a difference in the readings?
The person at Mouser picking parts hates me. $70 worth of tiny Dale resistors, mix-n-match caps, transistors, and ... stuff. Hoping the wima and nichicon caps are small enough to fit on the board without too much finagling.
Thats nothing...at work I will do a $300 order of parts like that almost every week. I'm sure the mouser people plan on sucky days everyday.
To be honnest I think its done by machines.
In case anyone needs to know oem values.
Fixing to chunk it into the trash can. Three days messing with it and still doa. Forgot to make the #2 cut from the tube center tap. Don't know if it was that or me being rough with the transistors (2n3906) but one died. Replaced both. Still not go. Looking at tube bias (from 220Ω resistor to ground) shows 22 volts (should be around 15) and the 5k trim pots move it less than a volt. Checked the trim pots with dats and they work as designed. Replaced the trim pots with oem 3k values anyway. Same results. Guess I'll add some lm317at regulators and irl530 mosfets on the next Mouser order.
Also checked and checked again all solder joints.
Photo from https://rockgrotto.proboards.com/thread/5297
Lead free solder and these cheap boards with tiny thin traces suk.
No dice. Going to fix it with a BFH.
Wait ...
Double triple check. Well, didn't measure the original resistors as they were pulled. The board says 25k √, 3k √, 1.5k √, etc. ... wait ... some of these resistors measure 7.5∩ not 7.5k, same with the 47∩ not 47k.
Dagnabbit!
A few resistor changes and you might be good to go. Good luck.
Looking at your photo above, 7R5 = 7.5 ohm. Common practice with schematics that are often photocopied, the R in place of the dot is easy to read, where dust/dirt on a photocopy could be misinterpreted.
It's alive!!! Let it warm up and set the bias at 15.3v on both pots. It sounds waaaaaay different that what I remember. I'll have to dig the other one out and listen before making too many comments.
And yeah, learned a valuable lesson on reading schematics. Could of had this thing up and running days ago if I'd just payed more attention.
Think you're going to like this @kenrhodes.
Bottom side.
The crosstalk mod consists of cutting the traces where circled and running a jumper. The traces on this board are not the best. The short blue wire on top and the longer green wire on the right are jumpers for suspect traces. Had to run the soldering iron over 400* to melt the lead-free oem solder. Carefully drilled two small 1/8th" holes in the easily broken plexi cover over the (as the picture sits) right side pins of the 220∩ resistor. This gives easy access for setting bias.
Replaced the imitation caps with Nichicon. In another old thread, the designer/manufacturer/whatever recommended not going over 6800uf and downsizing would be fine. The oem caps are rated at 25v, the power supply is 24v. Replaced the 6800uf with 4700uf/35v/105c and the two 1000uf/24v with 2200uf/35v/105c. Those lm317 get nice and toasty and one of those caps sits just millimeters away. Drilled more holes in the top plate to increase air movement over those toasty mosfets and to easily access the trim pots for adjusting bias.
They are a few non-Dale resistors in there. Those are the ones where I screwed up at reading schematics101. They will eventually be replaced with Dales when placing the next Mouser order. The input caps are Wima. According to some of the old threads, these input caps can be bypassed. At 1uf, I have to wonder if they might have some affect on the very bottom end. Switches/inputs/plugs/attenuation are all oem. The trim pots are 5k and one outside leg will need severed. The middle leg must remain. Keep in mind to trim the legs identically if the identical direction of adjustment is desired.
Parts list.
Mods1= largest difference in sound/putting a 25v cap in a 24v power supply. Mods2= wash-n-wax. Replace= what was replaced due to a blown tube and me not having the ability to troubleshoot electronics, or as far as that goes, not knowing jack schitt about electronics. The parts are relatively inexpensive and it was easier to replace them than trying to learn how to troubleshoot. Apparently, the red led was toast and it is an integral part of the circuit.
Prolly considerable interest for this mod on TT, I'll also post it there.
Kinda question your concern with " running the temps over 400" Your temps should be 6-700 degrees. You want quick melt / time for proper soldering. Running that low of a temp heats up the components and traces your soldering to much while waiting to create a solder joint. You want quick in quick out.